BackupPC_ovz: increase snapshot size

Mail server writes nearly 2GB during the backup window, so the previous
setting of 1GB for the snapshot partition is too small.  Because the
vecommon partition holds pretty much all the CTs, then the cumulative
write requirements of al CT's on that node using vecommon must be taken
into account when properly sizing the snapshot volume size.

25GB seems like a reasonable number for now.  That is at the moment
about 10% of the free space in the volume group.