qemu-iotests: Test the reopening of overlay_bs in 'block-commit'

qemu-iotests: Test the reopening of overlay_bs in 'block-commit'

The 'block-commit' command needs the overlay image of 'top' to
be opened in read-write mode in order to update the backing file
string. If 'top' is not the active layer or its backing file then its
overlay needs to be reopened during the block job.

This is a test case for that scenario.

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