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 form of this capability: The master expanded
the size after a reboot and slaves needed to be rebooted after that to make use
the of new space. But this functionality recently broke in trunk, as pvresize
now requires a lvm.conf with write-access. It also neer was a documented or
tested functionality.

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