Avoid crashing on multiple -incoming

Passing multiple -incoming options used to crash qemu (due to
an invalid state transition incoming->incoming).  Instead we now
take the last -incoming option, e.g.:

qemu-system-x86_64 -nographic -incoming tcp::4444 -incoming defer

ends up doing the defer.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
