virStorageSource: Add fields for storing last I/O error message

virStorageSource: Add fields for storing last I/O error message

Hypervisors may report a I/O error message (unstable; for human use)
to libvirt. In order to store it with the appropriate virStorageSource
so that it can be later queried we need to add fields to
virStorageSource to store the timestamp and message.

The message is deliberately not copied via virStorageSourceCopy.

The messages are also not serialized to the status XML as losing them on
a daemon restart as they're likely to be stale anyways.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>