storvsc: in responce to a scan event, scan the host

The virtual HBA that storvsc implements can support multiple channels and
targets. So, scan the host when the host notifies that a scan is needed.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>