qemu-iotests: add initial tests for live block commit

Derived from the streaming test cases (030), this adds the
following 9 tests:

1. For the following image chain, commit [mid] into [backing],
   and use qemu-io to verify [backing] has its original data, as
   well as the data from [mid]

           [backing] <-- [mid] <-- [test]

2. Verifies that 'block-commit' with the 'speed' parameter sets the
   speed parameter, as reported by 'query-block-jobs'

3. Verifies that a bogus 'device' parameter to 'block-commit'
   results in error

4-9: Appropriate error values returned for the following argument errors:
    * top == base
    * top is nonexistent
    * base is nonexistent
    * top == active layer (this is currently not supported)
    * top and base arguments are reversed
    * top argument is omitted

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
