monitor: restrict command getfd to POSIX hosts

Currently, the function will simply fail if ancillary fds are not
provided, for ex on unsupported platforms.

This changes the failure from:

    {"error": {"class": "GenericError", "desc": "No file descriptor
    supplied via SCM_RIGHTS"}}

to:

    {"error": {"class": "CommandNotFound", "desc": "The command getfd
    has not been found"}}

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

hmp-commands.hx

@@ -1486,6 +1486,7 @@ SRST
   Inject an MCE on the given CPU (x86 only).
 ERST
 
+#ifdef CONFIG_POSIX
     {
         .name       = "getfd",
         .args_type  = "fdname:s",
@@ -1501,6 +1502,7 @@ SRST
   mechanism on unix sockets, it is stored using the name *fdname* for
   later use by other monitor commands.
 ERST
+#endif
 
     {
         .name       = "closefd",