xen: use the 'allocating' style of 'gntshr' (aka gntmap)

The new mini-os APIs are allocating in the sense that they return
fresh buffers containing foreign grant-mapped pages rather than
requiring us to pass in our own buffers.

One day everything will be this style and we can get rid of the
dead code.

Signed-off-by: David Scott <dave.scott@citrix.com>