xen: Initialize event channels and io rings

Open and bind event channels; map ioreq and buffered ioreq rings.

Signed-off-by: Arun Sharma <arun.sharma@intel.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
