xen_disk: fix unmapping of persistent grants

This patch fixes two issues with persistent grants and the disk PV backend
(Qdisk):

 - Keep track of memory regions where persistent grants have been mapped
   since we need to unmap them as a whole. It is not possible to unmap a
   single grant if it has been batch-mapped. A new check has also been added
   to make sure persistent grants are only used if the whole mapped region
   can be persistently mapped in the batch_maps case.
 - Unmap persistent grants before switching to the closed state, so the
   frontend can also free them.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
