tests: qemublock: Add basic 'raw' file test

tests: qemublock: Add basic 'raw' file test

Test the JSON props generator with a very simple 'raw' image with no
other options. The node-names for the image are 31 bytes long so that we
validate our node name detector.

The top level disk image would generate the following '-drive' cmdline:

-drive file=/var/lib/libvirt/images/i.img,format=raw,if=none,id=drive-dummy
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>