xenbus: Use grant-table wrapper functions

For xenbus_{map,unmap}_ring to work on HVM, the grant table operations
must be set up using the gnttab_set_{map,unmap}_op functions instead of
directly populating the fields of gnttab_map_grant_ref. These functions
simply populate the structure on paravirtualized Xen; however, on HVM
they must call __pa() on vaddr when populating op->host_addr because the
hypervisor cannot directly interpret guest-virtual addresses.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
[v1: Fixed cleanpatch error]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>