libxc: Introduce xc_domain_nr_gpfns as a cousin of xc_domain_maximum_gpfn.

libxc: Introduce xc_domain_nr_gpfns as a cousin of xc_domain_maximum_gpfn.

The commit a8f8a590e02d2d2b717257c0bd9a8b396103bdf4
"libxc: Check xc_domain_maximum_gpfn for negative return values"
introduced an regression in tools outside libxc (migrate v2)
which wanted the unfiltered GPFN value. Said commit added
a wrapper which added +1 if there were no errors.

To make it work pre-commit a8f8a59 we add an xc_domain_nr_gpfns
which will add +1 if there are no errors (and change all in-tree
callers to use it). The xc_domain_maximum_gpfn will return the
unfiltered GPFN value.

Suggested-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>