Disallow outward migration while awaiting incoming migration

QEMU will assert if you attempt to start an outgoing migration on
a QEMU that's sitting waiting for an incoming migration (started
with -incoming), so disallow it with a proper error.

(This is a fix for https://bugzilla.redhat.com/show_bug.cgi?id=1086987 )

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
