qcow2: Report error for version > 2

The qcow2 driver is now declared responsible for any QCOW image that has
version 2 or greater (before this, version 3 would be detected as raw).

For everything newer than version 2, an error is reported.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>