Stick to the convention and use bytes as the unit for bd_lvm_pvresize

The default for the pvresize utility is MB, but the whole library and the LVM
plugin both use B as a unit.
Make bd_lvm_pvresize usable for more (common) cases

It's quite common to just request the PV to be resized to the block device's
(new) size.

Also fix the memory leak.