qemublocktest: Use "target only" mode in 'testJSONtoJSON' and 'testBackingXMLjsonXML'

qemublocktest: Use "target only" mode in 'testJSONtoJSON' and 'testBackingXMLjsonXML'

Both tests pass a disk source definition which didn't go through the
preparation steps and thus contains only the target information that
were originally present, thus we should be using the
QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_TARGET_ONLY flag.

For the same reason QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_AUTO_READONLY
used in 'testJSONtoJSON' doesn't make sense.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>