Staging: hv: Introduce a common function for issuing commands to the device

There are multiple functions to either get info about the device
(INQUIRY, CAPACITY etc) or to FLUSH the device cache. The majority of the
code for these functions is identical. Introduce a common function for
doing all these operations.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>