Code move of vg_mark_partial() up in stack

Code move of vg_mark_partial() up in stack

It's useful to keep the partial flag cached - so just move the call
for vg_mark_partil_lvs() into import_vg_from_config_tree() so it gets
evaluated before it goes through the lvmcache.

This patch should not present any functional change.

Note: It is rather temporal solution - proper place is probably inside the
'read' call back - but needs some more discussion.
For now using this minor hack.