xen/blkback: Free resources if connect_ring failed.

xen/blkback: Free resources if connect_ring failed.

With the multi-queue support we could fail at setting up
some of the rings and fail the connection. That meant that
all resources tied to rings[0..n-1] (where n is the ring
that failed to be setup). Eventually the frontend will switch
to the states and we will call xen_blkif_disconnect.

However we do not want to be at the mercy of the frontend
deciding when to change states. This allows us to do the
cleanup right away and freeing resources.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 2d0382fac17cef20d507a0211b82e0942b2ab271)
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>