xen: cleanup IOREQ server on exit

Device model is supposed to destroy IOREQ server for itself.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Acked-by: Paul Durrant <paul.durrant@citrix.com>
Message-Id: <1564428563-1006-1-git-send-email-igor.druzhinin@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>