xen, fbfront: fix connecting to backend

commit 9121b15b5628b38b4695282dc18c553440e0f79b 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: Willy Tarreau <w@1wt.eu>
Signed-off-by: Corey Edwards <ensabahnur16@gmail.com>