xen/blkfront: avoid compiler warning from missing cases

Fix:
drivers/block/xen-blkfront.c: In function ‘blkfront_connect’:
drivers/block/xen-blkfront.c:933: warning: enumeration value ‘BLKIF_STATE_DISCONNECTED’ not handled in switch

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>