Fix lvm.extend_volume to pass Gig suffix

The extend function in the lvm driver was not converting
the cinder size value to Gigabytes before passing the call
to the vg module.  The result was that we would attempt to
extend a volume to "new size in Megabytes" which of course
is less than the current size since we do a Gigabyte string
conversion on create and everywhere else.

This change makes sure we pass the integer change through
the sizestr method to get the G suffix needed to work properly.

Change-Id: I070962a3aa7038f612e19a93ccaa60cbc13008f6
Closes-Bug: #1240287
(cherry picked from commit ec442e41d2d243003a42ed60ae862e96142a5cde)