When running the xen-save-devices-state QMP command, if the filename
already exists it will be truncated before dumping the devices' state
into it.

Signed-off-by: Dov Murik <dovmurik@linux.vnet.ibm.com>
Message-Id: <20200921094830.114028-1-dovmurik@linux.vnet.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>