rename qcow3 to qcow2_v3

rename qcow3 to qcow2_v3 and disable it on unsupported hosts

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
Merge pull request #630 from xiaoqing-wei/master

rename qcow3 to qcow2_v3 and disable it on unsupported hosts