SR.ls: pick up the LV size in 'lvs'

Signed-off-by: David Scott <dave.scott@citrix.com>
SR.ls uses a common 'volume_of_lv' function

Signed-off-by: David Scott <dave.scott@citrix.com>
Volume.stat uses 'lvs'

Signed-off-by: David Scott <dave.scott@citrix.com>