Staging: hv: Move the definition of struct storvsc_device

In preparation for getting rid of the inclusion of storvsc.c from
blkvsc.c, move the definition of struct storvsc_device
from storvsc.c to storvsc_api.h.

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>