QEMU requires an extra wrapper object where only the "file" member is
populated. This is basically a placeholder for establishing the format
layer. We did the same in qemuDiskSourceGetProps for the old-school
JSON usage with -drive but forgot to adopt this for -blockdev.
https://bugzilla.redhat.com/show_bug.cgi?id=1804617
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
