vdo: enhance detection of virtual size

vdo: enhance detection of virtual size

Improve detection of VDO virtual size - so it's not reading VDO
metadata when VDO device is already active and instead we reuse
existing table line for knowing existing metadata size.

NOTE: if there is ever going to be added support for reduction
of VDO virtual size - this method will need to be reworked to
allow size difference only within 'extent_size' alignment.