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 on 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 new space. But this functionality was broken in trunk, as
pvresize now requires a lvm.conf with write-access.

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