tests: qemublock: Add test-case for the 'vvfat' driver in qemu

tests: qemublock: Add test-case for the 'vvfat' driver in qemu

Test mapping of the 'FAT' disk format to 'vvfat' in qemu.

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

dir-fat-readonly.xml:
-drive file=fat:/var/somefiles,if=none,id=drive-dummy,readonly=on
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

dir-fat-floppy.xml
-drive file=fat:floppy:/var/somefiles,if=none,id=drive-dummy,readonly=on

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