Replace disks: keep the meta device in the same VG

This patch enhances the multi-VG support in replace disks, by keeping
the meta device in the same VG, as opposed to moving it to the data
device VG (note that we don't have a way to create the meta in a
different VG in the first place, but at least we correctly handle a
custom config).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>