The _free_vg that is created as a placeholder when reporting segments in pvs

The _free_vg that is created as a placeholder when reporting segments in pvs
was lacking the (vgmem) pool. We now create that pool. There is at least one
more such VG (_dummy_vg) which is pool-less. I am not sure what is the right
way to go about this, but this is currently necessary to fix a segfault
introduced by using vgmem in the reporter in Dave's lvseg lvm2app patches.

Signed-off-by: Petr Rockai <prockai@redhat.com>