libxc: introduce xc_map_domain_meminfo (and xc_unmap_domain_meminfo)

libxc: introduce xc_map_domain_meminfo (and xc_unmap_domain_meminfo)

And use it in xc_exchange_page(). This is basically because the
following change need something really similar to the set of
steps that are here abstracted in these two functions.

Despite of the change in the interface and in the signature of
some functions, this is pure code motion. No functional changes
involved.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>