libxc: introduce xc_domain_get_guest_width()

libxc: introduce xc_domain_get_guest_width()

As a wrapper to XEN_DOMCTL_get_address_size, and use it
wherever the call was being issued directly via do_domctl(),
saving quite some line of code.

Actually, the function returns the guest width in bytes,
rather than directly what XEN_DOMCTL_get_address_size provides
(which is a number of bits), since that is what it is useful
almost everywhere.

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