Probe as the same behavior as VMware does.
Recognize image as monolithicFlat descriptor file when the file is text
and the first effective line (not '#' leaded comment or space line) is
either 'version=1' or 'version=2'. No space or upper case charactors
accepted.
Signed-off-by: Fam Zheng <famcool@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>