Drivers: hv: Add CONFIG_HYPERV_VMBUS option

Drivers: hv: Add CONFIG_HYPERV_VMBUS option

At present VMBus driver is hinged off of CONFIG_HYPERV which entails
lot of builtin code and encompasses too much. It's not always clear
what depends on builtin hv code and what depends on VMBus. Setting
CONFIG_HYPERV as a module and fudging the Makefile to switch to builtin
adds even more confusion. VMBus is an independent module and should have
its own config option. Also, there are scenarios like baremetal dom0/root
where support is built in with CONFIG_HYPERV but without VMBus. Lastly,
there are more features coming down that use CONFIG_HYPERV and add more
dependencies on it.

So, create a fine grained HYPERV_VMBUS option and update Kconfigs for
dependency on VMBus.

Signed-off-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>	# drivers/pci
Signed-off-by: Wei Liu <wei.liu@kernel.org>