xen/pvh: /dev/xen/privcmd changes.

xen/pvh: /dev/xen/privcmd changes.

PVH only supports the batch interface. To map a foreign page to a process,
the PFN must be allocated and PVH path uses ballooning for that purpose.

The returned PFN is then mapped to the foreign page.
xen_unmap_domain_mfn_range() is introduced to unmap these pages via the
privcmd close call.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
[v1: Fix up privcmd_close]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>