Staging: hv: vmbus: Support building the vmbus driver as part of the kernel

Modify the way we initialize the vmbus driver so that all the hyper-v drivers
can be linked with the kernel and still ensure that the vmbus driver
is fully initialized before the drivers that depend upon the vmbus
driver attempt to initialize.

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>