monitor: Enable adding an inherited fd to an fd set

qmp_add_fd() gets an fd that was received over a socket with
SCM_RIGHTS and adds it to an fd set.  This patch adds support
that will enable adding an fd that was inherited on the
command line to an fd set.

Note: All of the code added to monitor_fdset_add_fd(), with the
exception of the error path for non-valid fdset-id, is code motion
from qmp_add_fd().

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
