qemu: snapshot: Allow snapshots of read-only disks when we can create them

qemu: snapshot: Allow snapshots of read-only disks when we can create them

With -blockdev or when reusing externally created images and thus
without the need for formatting the image we actually can support
snapshots of read-only disks. Arguably it's not very useful so they are
not done by default but users of libvirt such as oVirt are actually
using this.

https://bugzilla.redhat.com/show_bug.cgi?id=1832204

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