Refactors byte size extraction logic

The logic of _extract_bytes() in imageutils.QemuImgInfo
is not efficient. See
https://review.openstack.org/#/c/41363/11/openstack/common/imageutils.py

This patch enhances the regex pattern to extract the byte size info
more efficiently.

Change-Id: I6efb3a6020179ac93c9d7848403c0fad0ebb1d1c
Merge "Refactors byte size extraction logic"