vdo: deactivate forgotten vdo pool

vdo: deactivate forgotten vdo pool

If the linear mapping is lost (for whatever reason, i.e.
test suite forcible  'dmsetup remove' linear LV,
lvm2 had hard times figuring out how to deactivate such DM table.

So add function which is in case inactive VDO pool LV checks if
the pool is actually still active (-vpool device present) and
it has open count == 0.  In this case deactivation is allowed
to continue and cleanup DM table.