If a -drive has no image, using image properties makes qemu whine that
they should not be used.

This patch stops formating cache/readonly/... for empty drives
for the pre-blockdev syntax. Unfortunately those parameters can't be
added later when inserting media, but on the other hand qemu will start
with an empty drive.

Since we already were able to start a VM with such config previously due
to qemu ignoring them I've opted just to skip formatting them.
Additionally with -blockdev support it will work as expected as the
image properties will be formatted when adding the image itself which is
not possible without it.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1651457Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>