Can't build in the virtio console driver on x86_64 when the other virtio drivers are modular

[Chuck Ebbert <cebbert@redhat.com>]

The virtio configuration options are inconsistent. According to this,
every options that needs virtio will select it:

 # Virtio always gets selected by whoever wants it.
 config VIRTIO
        tristate

Note that it's not user-selectable, so any config file that tries to
set it will be ignored when kconfig loads those options. And yet we
have a whole set of options that depend on VIRTIO, like VIRTIO_CONSOLE
for example. This makes it impossible to have VIRTIO_PCI modular and
VIRTIO_CONSOLE built-in on x86_64, because:

1. VIRTIO_PCI selects VIRTIO and sets it to M
2. VIRTIO_CONSOLE gets forced to M because one of its dependencies is M

So either VIRTIO (and VIRTIO_RING) need to be user-selectable, or all
of the options that depend on VIRTIO need to be changed to select it
instead.

(For even more fun, LGUEST_GUEST on i386 forces VIRTIO, VIRTIO_RING and
VIRTIO_CONSOLE all to Y.)