Fix convert polling to ignore LV with different UUID.

Fix convert polling to ignore LV with different UUID.

When mirror convert polling is started (mainly as backgound process,
in lvchange -a y or in lvconvert itself) it tries to read VG
and LV identified by its name.

Unfortunatelly, the VG can have already different LV under the same name,
and various more or less funny things can happen (note that
_finish_lvconvert_mirror suspends the volume for example).

(the typical example is our testing script which continuously recreates
LVs under the same name in the same VG.)

This patch adds optional uuid parameter which helps to properly
select the monitoring object. For lvconvert polling it is set to LV UUID
and both _get_lvconvert_vg and _get_lvconvert_lv uses it to read proper VG/LV.

(In the pvmove case it is NULL, here we poll for physical volume name).