libxc: unify xc_dom_p2m_{host/guest}

libxc: unify xc_dom_p2m_{host/guest}

Unify both functions into xc_dom_p2m. Should not introduce any functional
change.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>