block: Disallow snapshots if the overlay doesn't support backing files
This addresses scenarios like this one:
{ 'execute': 'blockdev-add', 'arguments':
{ 'options': { 'driver': 'qcow2',
'node-name': 'new0',
'file': { 'driver': 'file',
'filename': 'new.qcow2',
'node-name': 'file0' } } } }
{ 'execute': 'blockdev-snapshot', 'arguments':
{ 'node': 'virtio0',
'overlay': 'file0' } }
Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>