xen, fbfront: fix connecting to backend

commit 9121b15b upstream.

Connecting to the backend isn't working reliably in xen-fbfront: in
case XenbusStateInitWait of the backend has been missed the backend
transition to XenbusStateConnected will trigger the connected state
only without doing the actions required when the backend has
connected.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>