iotests/118: Add -blockdev based tests

iotests/118: Add -blockdev based tests

The code path for -device drive=<node-name> or without a drive=...
option for empty drives, which is supposed to be used with -blockdev
differs enough from the -drive based path with a user-owned
BlockBackend, so we want to test both paths at least for the basic tests
implemented by TestInitiallyFilled and TestInitiallyEmpty.

This would have caught the bug recently fixed for inserting read-only
nodes into a scsi-cd created without a drive=... option.

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