postcopy: Allow shared memory

postcopy: Allow shared memory

Now that we have the mechanisms in here, allow shared memory in a
postcopy.

Note that QEMU can't tell who all the users of shared regions are
and thus can't tell whether all the users of the shared regions
have appropriate support for postcopy.  Those devices that explicitly
support shared memory (e.g. vhost-user) must check, but it doesn't
stop weirder configurations causing problems.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>