Enable online expansion of LVHDxSRs

Expand the size of LVHDxSRs by growing into additional availiable space, if
extra space is availiable on the backing device during sr-attach of the master
or sr-scan.

Up to Clearwater we had a limited undocumented form of this capability: Offline
expansion. The master expanded the size after a reboot and slaves needed to be
rebooted after that to make use the of new space.

Signed-off-by: Robert Breker <robert.breker@citrix.com>