Drivers: hv: vmbus: skip VMBus initialization if Linux is root

There is no VMBus and the other infrastructures initialized in
hv_acpi_init when Linux is running as the root partition.

Signed-off-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210203150435.27941-4-wei.liu@kernel.org