libvirt: video RAM setting should be passed in kb to libvirt

Currently, video RAM setting is being set in MB in both
flavor extra_spec and the image property, however, it is not
being translated to KB, in the instance XML config.

Change-Id: Ie1091c08a011d650a7bb0ff0337337ceae7b111b
Closes-Bug: #1373239
Merge "libvirt: video RAM setting should be passed in kb to libvirt"