libxc: make arch_setup_meminit a xc_dom_arch hook

libxc: make arch_setup_meminit a xc_dom_arch hook

This allows having different arch_setup_meminit implementations based on the
guest type. It should not introduce any functional changes.

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>