Support guest shutdown of Alpine Linux in guest agent
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
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) {
```