ZFSPoolPlugin: fix #2662 get volume size correctly

ZFSPoolPlugin: fix #2662 get volume size correctly

Getting the volume sizes as byte values instead of converted to human
readable units helps to avoid rounding errors in the further processing
if the volume size is more on the odd side.

The `zfs list` command supports the -p(arseable) flag since a few years
now.
When returning the size in bytes there is no  calculation performed and
thus we need to explicitly cast the size to an integer before returning
it.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>