migration/postcopy: Explicitly disallow huge pages

migration/postcopy: Explicitly disallow huge pages

At the moment postcopy will fail as soon as qemu tries to register
userfault on the RAMBlock pages that are backed by hugepages.
However, the kernel is going to get userfault support for hugepage
at some point, and we've not got the rest of the QEMU code to support
it yet, so fail neatly with an error like:

Postcopy doesn't support hugetlbfs yet (/objects/mem1)

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>