storage_driver: Mark volume as 'in use' for some operations

storage_driver: Mark volume as 'in use' for some operations

There are few operations in the storage driver that read/write
data onto volumes. Such operations can take very long time to
finish. During that time the storage pool object is locked which
has bad performance impacts (other threads can't fetch its XML
for instance). This commit prepares the storage driver for
releasing the lock during those operations (downloadVol,
uploadVol, wipeVol).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>