libxc/migration: Drop unimplemented domain types

libxc/migration: Drop unimplemented domain types

x86 PVH is completely obsolete - it was intended for legacy PVH before that
idea was abandoned.  There was an RFC series for ARM in 2015, but there is
plenty of outstanding work which hasn't been done yet.

No functional change.  New types can be (re)introduced with the code which
actually implements them.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien@xen.org>
Acked-by: Wei Liu <wl@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>