qemu-iotests: Rewrite 212 for blockdev-create job

This rewrites the test case 212 to work with the new x-blockdev-create
job rather than the old synchronous version of the command.

All of the test cases stay the same as before, 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>