Drivers: hv: Make the vmbus driver unloadable

It turns out that the vmbus driver can be made unloadable. Make it
unloadable.

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>