qemu: block: Use simple backing stores string format if possible

qemu: block: Use simple backing stores string format if possible

In case when the backing store can be represented with something
simpler such as a URI we can use it rather than falling back to the
json: pseudo-protocol.

In cases when it's not worth it (e.g. with the old ugly NBD or RBD
strings) let's switch to json.

The function is exported as we'll need it when overwriting the ugly
strings qemu would come up with during blockjobs.

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