iotests: Add test for multiple BB on BDS tree

This adds a test for having multiple BlockBackends in one BDS tree. In
this case, there is one BB for the protocol BDS and one BB for the
format BDS in a simple two-BDS tree (with the protocol BDS and BB added
first).

When bdrv_close_all() is executed, no cached data from any BDS should be
lost; the protocol BDS may not be closed until the format BDS is closed.
Otherwise, metadata updates may be lost.

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