CAR-1368: Enable online expansion of LVHDxSRs

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

This commit superseeds the limited undocumented functionality to expand LVHDxSRs
which we had before. The master used to do the resize after a reboot during
sr-attach. Pool members needed to be rebooted after the master.

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