Fix panic in stop when the VM goes away

[alexandear]

Fixes #102

The panic occurs because the closed channel haStdoutTail.Lines returns nil when the file haStdoutTail is removed.

The test has not been added because writing it without time.Sleep is challenging.

Reproducing the problem locally

Before the PR

❯ _output/bin/limactl delete -f default
...

❯ _output/bin/limactl stop default
INFO[0000] Sending SIGINT to hostagent process 37797    
INFO[0000] Waiting for the host agent and the driver processes to shut down 
INFO[0000] [hostagent] Received SIGINT, shutting down the host agent 
INFO[0000] [hostagent] Shutting down the host agent     
INFO[0000] [hostagent] Stopping forwarding "/run/lima-guestagent.sock" (guest) to "/Users/Oleksandr_Redko/.lima/default/ga.sock" (host) 
INFO[0000] [hostagent] Unmounting "/Users/Oleksandr_Redko" 
INFO[0000] [hostagent] Unmounting "/tmp/lima"           
INFO[0000] [hostagent] Shutting down QEMU with ACPI     
INFO[0000] [hostagent] Sending QMP system_powerdown command 
2024/07/09 22:52:14 Stopping tail as file no longer exists: /Users/Oleksandr_Redko/.lima/default/ha.stdout.log
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x40 pc=0x10521f66c]

goroutine 1 [running]:
github.com/lima-vm/lima/pkg/hostagent/events.Watch({0x1059712e8, 0x140004e5490}, {0x14000552200?, 0x106d57ac8?}, {0x14000552240, 0x32}, {0x30?, 0x14000089b08?, 0x10626aa20?}, 0x14000427b50)
        /Users/Oleksandr_Redko/src/github.com/lima-vm/lima/pkg/hostagent/events/watcher.go:47 +0x1ec
main.waitForHostAgentTermination({0x105971160?, 0x1067fc940?}, 0x1400047d680, {0x1?, 0x1?, 0x10626aa20?})
        /Users/Oleksandr_Redko/src/github.com/lima-vm/lima/cmd/limactl/stop.go:96 +0x13c
main.stopInstanceGracefully(0x1400047d680)
        /Users/Oleksandr_Redko/src/github.com/lima-vm/lima/cmd/limactl/stop.go:74 +0x1d0
main.stopAction(0x14000341508, {0x140004f2a00?, 0x4?, 0x1054c8577?})
        /Users/Oleksandr_Redko/src/github.com/lima-vm/lima/cmd/limactl/stop.go:53 +0xd0
github.com/spf13/cobra.(*Command).execute(0x14000341508, {0x140004f29d0, 0x1, 0x1})
        /Users/Oleksandr_Redko/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:985 +0x840
github.com/spf13/cobra.(*Command).ExecuteC(0x14000340c08)
        /Users/Oleksandr_Redko/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1117 +0x344
github.com/spf13/cobra.(*Command).Execute(...)
        /Users/Oleksandr_Redko/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1041
main.main()
        /Users/Oleksandr_Redko/src/github.com/lima-vm/lima/cmd/limactl/main.go:27 +0x20

After the PR

❯ _output/bin/limactl delete -f default
...

❯ _output/bin/limactl stop default
INFO[0000] Sending SIGINT to hostagent process 34969    
INFO[0000] Waiting for the host agent and the driver processes to shut down 
INFO[0000] [hostagent] Received SIGINT, shutting down the host agent 
INFO[0000] [hostagent] Shutting down the host agent     
INFO[0000] [hostagent] Stopping forwarding "/run/lima-guestagent.sock" (guest) to "/Users/Oleksandr_Redko/.lima/default/ga.sock" (host) 
INFO[0000] [hostagent] Unmounting "/Users/Oleksandr_Redko" 
INFO[0000] [hostagent] Unmounting "/tmp/lima"           
INFO[0000] [hostagent] Shutting down QEMU with ACPI     
INFO[0000] [hostagent] Sending QMP system_powerdown command 
2024/07/09 22:49:25 Stopping tail as file no longer exists: /Users/Oleksandr_Redko/.lima/default/ha.stdout.log
FATA[0000] did not receive an event with the "exiting" status 

[jandubois]

Thanks, LGTM