storvsc: add logging for error/warning messages

Introduce a logging level for storvsc to log certain error/warning
messages. Those messages are helpful in some environments,
e.g. Microsoft Azure, for customer support and troubleshooting purposes.

Signed-off-by: Long Li <longli@microsoft.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>