Staging: hv: storvsc: Introduce state to manage the lifecycle of stor device

Introduce state to manage the lifecycle of stor device. This would be the
basis for managing the references on the stor 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>