Experiment with using pvresize to allow users to resize LVHDxSRs online

Expands the size of the SR 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.

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