Storage: ZFS: implement `resizeVol` method to support native resize

Storage: ZFS: implement `resizeVol` method to support native resize

ZFS doesn't have thick allocations, every allocation is
thin-provisioned, so resize operation is essentially
a zvol size limit change
(`zfs set volsize=X pool/zvol_name`).

Shrink is allowed too (which leads to data destruction),
but shrink restriction is already implemented in libvirt,
so this function doesn't need to check anything.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>