Staging: hv: storvsc: Eliminate the usage of ext field in struct hv_device

Now, eliminate the usage of ext field in struct  hv_device for storvsc driver.
We do this by registering pointer to struct storvsc_device as the driver
specific data and eliminating the current usage of driver specific data to
save and retrieve the pointer to struct Scsi_Host.
Additionally, all access to the driver specific data is through
the vmbus wrapper functions. Note that function to allocate the host
gives us a reference on the host object.

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