migration/postcopy: Detect file system on dest host

Postcopy requires the memory support userfaultfd to work.  Right now we
check it but it's a bit too late (when switching to postcopy migration).

Do that early right at enabling of postcopy.

Note that this is still only a best effort because ramblocks can be
dynamically created.  We can add check in hostmem creations and fail if
postcopy enabled, but maybe that's too aggressive.

Still, we have chance to fail the most obvious where we know there's an
existing unsupported ramblock.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

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