storage: Add writelabel bool for virStorageBackendDeviceProbe

storage: Add writelabel bool for virStorageBackendDeviceProbe

It's possible that the API could be called from a startup path in
order to check whether the label on the device matches what our
format is. In order to handle that condition, add a 'writelabel'
boolean to the API in order to indicate whether a write or just
read is about to happen.

This alters two "error" conditions that would care about knowing.

Signed-off-by: John Ferlan <jferlan@redhat.com>