drivers: xen-blkback: delay pending_req allocation to connect_ring

drivers: xen-blkback: delay pending_req allocation to connect_ring

This is a pre-patch for multi-page ring feature.
In connect_ring, we can know exactly how many pages are used for the shared
ring, delay pending_req allocation here so that we won't waste too much memory.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 69b91ede5cab843dcf345c28bd1f4b5a99dacd9b)
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>