Drivers: hv: Enable vmbus driver for nested root partition

Currently VMBus driver is not initialized for root partition but we need
to enable the VMBus driver for nested root partition. This is required,
so that L2 root can use the VMBus devices.

Signed-off-by: Jinank Jain <jinankjain@linux.microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/c3cdd2cf2bffeba388688640eb61bc182e4c041d.1672639707.git.jinankjain@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>