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>