Use free space in vg instead of biggest free pv space for a snapshot

Even for snapshot we looked at the biggest free pv space even though
the vg might have fit the snapshot we aborted if one of the pvs was too
small. This patch fixes this by looking at the vg size instead of the pv
size.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>