monitor: fix qmp_getfd() fd leak in error case

qemu_chr_fe_get_msgfd() transfers ownership of the file descriptor to
the caller.  Therefore all code paths in qmp_getfd() should either
register the file descriptor somewhere or close it.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
