storvsc: Use the specified target ID in device lookup

The current code assumes that there is only one target in device lookup.
Fix this bug. This will alow us to correctly handle hot reomoval of
LUNs.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Alex Ng <alexng@microsoft.com>
Tested-by: Vivek Yadav <vyadav@microsoft.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>