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=1832204Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -14687,7 +14687,7 @@ qemuDomainSnapshotPrepareDiskExternal(virDomainObjPtr vm,
     int err;
     int rc;
 
-    if (disk->src->readonly) {
+    if (disk->src->readonly && !(reuse || blockdev)) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("external snapshot for readonly disk %s "
                          "is not supported"), disk->dst);