Support guest shutdown of Alpine Linux in guest agent

Host environment

• Operating system: n/a
• OS/kernel version: n/a
• Architecture: n/a
• QEMU flavor: n/a
• QEMU version: all
• QEMU command line: n/a

Emulated/Virtualized environment

• Operating system: Alpine Linux
• OS/kernel version: n/a
• Architecture: all

Description of problem

The qemu-guest-agent's shutdown calls /sbin/shutdown with the apropriate flags to shut down a posix system. On Alpine Linux, which is based on busybox, there is no /sbin/shutdown, instead there are /sbin/poweroff, /sbin/halt and /sbin/reboot. We have used a downstream patch for years that will exec those as a fallback in case execing /sbin/shutdown fails.

With qemu 9.2 this patch no longer applies and it is probably time to solve this properly in upstream qemu.

The question is how?

Some options:

• Set the powerdown, halt and reboot commands via build time configure option
• Add a fallback if the execlp fails (similar to what downstream Alpine's patch does now). We could for example give ga_run_command a const char **argv[], and try execvp all of them before erroring out.
• Test the existence of /sbin/shutdown before calling ga_run_command.
• Do nothing. Let downstream Alpine Linux handle it.

Steps to reproduce

1. Build qemu-guest-agent for Alpine Linux
2. boot a Alpine linux VM and install the qemu-guest-agent
3. Try shutdown the VM via qmp command.

Additional information

The patch that we previously used that no longer applies:

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 954efed01..61427652c 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -84,6 +84,7 @@ static void ga_wait_child(pid_t pid, int *status, Error **errp)
 void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
 {
     const char *shutdown_flag;
+    const char *fallback_cmd = NULL;
     Error *local_err = NULL;
     pid_t pid;
     int status;
@@ -101,10 +102,13 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
     slog("guest-shutdown called, mode: %s", mode);
     if (!has_mode || strcmp(mode, "powerdown") == 0) {
         shutdown_flag = powerdown_flag;
+        fallback_cmd = "/sbin/poweroff";
     } else if (strcmp(mode, "halt") == 0) {
         shutdown_flag = halt_flag;
+        fallback_cmd = "/sbin/halt";
     } else if (strcmp(mode, "reboot") == 0) {
         shutdown_flag = reboot_flag;
+        fallback_cmd = "/sbin/reboot";
     } else {
         error_setg(errp,
                    "mode is invalid (valid values are: halt|powerdown|reboot");
@@ -125,6 +129,7 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
 #else
         execl("/sbin/shutdown", "shutdown", "-h", shutdown_flag, "+0",
                "hypervisor initiated shutdown", (char *)NULL);
+        execle(fallback_cmd, fallback_cmd, (char*)NULL, environ);
 #endif
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {