libxc: add xc_domain_add_to_physmap_batch to wrap XENMEM_add_to_physmap_batch

libxc: add xc_domain_add_to_physmap_batch to wrap XENMEM_add_to_physmap_batch

This is a preparation for the proposal "allow setting up shared memory areas
between VMs from xl config file". See:
V2: https://lists.xen.org/archives/html/xen-devel/2017-06/msg02256.html
V1: https://lists.xen.org/archives/html/xen-devel/2017-05/msg01288.html

The plan is to use XENMEM_add_to_physmap_batch in xl to map foregin pages from
one DomU to another so that the page could be shared. But currently there is no
wrapper for XENMEM_add_to_physmap_batch in libxc, so we just add a wrapper for
it.

Signed-off-by: Zhongze Liu <blackskygg@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>