xen_disk: add persistent grant support to xen_disk backend

xen_disk: add persistent grant support to xen_disk backend

This protocol extension reuses the same set of grant pages for all
transactions between the front/back drivers, avoiding expensive tlb
flushes, grant table lock contention and switches between userspace
and kernel space. The full description of the protocol can be found in
the public blkif.h header.

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob_plain;f=xen/include/public/io/blkif.h

Speed improvement with 15 guests performing I/O is ~450%.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>