pvmove: require LV name in a shared VG

pvmove: require LV name in a shared VG

In a shared VG, only allow pvmove with a named LV,
so that only PE's used by the LV will be moved.
The LV is then activated exclusively, ensuring that
the PE's being moved are not used from another host.

Previously, pvmove was mistakenly allowed on a full PV.
This won't work when LVs using that PV are active on
other hosts.