This rewrites the test case 207 to work with the new x-blockdev-create
job rather than the old synchronous version of the command.

Most of the test cases stay the same as before (the exception being some
improved 'size' options that allow distinguishing which command created
the image), but in order to be able to implement proper job handling,
the test case is rewritten in Python.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>