storage: Adjust calculation of alloc/capacity for disk

storage: Adjust calculation of alloc/capacity for disk

https://bugzilla.redhat.com/show_bug.cgi?id=1247987

Calculation of the extended and logical partition values for the disk
pool is complex. As the bz points out an extended partition should have
it's allocation initialized to 0 (zero) and keep the capacity as the size
dictated by the extents read.  Then for each logical partition found,
adjust the allocation of the extended partition.

Finally, previous logic tried to avoid recalculating things if a logical
partition was deleted; however, since we now have special logic to handle
the allocation of the extended partition, just make life easier by reading
the partition table again - rather than doing the reverse adjustment.