qemu: migration: Don't remember seclabel for images shared from current host

qemu: migration: Don't remember seclabel for images shared from current host

In case when the user exports images from current host and there is an
incoming migration from a remote host, security label remembering would
be possible but would attempt to remember the label allowing access to
the image as the image is already used by a VM on remote host.

To prevent remembering the wrong label, we'll skip the remembering of
the label for any shared resource, so that the code behaves identically
regardless of how the image is accessed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>