virtio-input: Fix device introspection on non-Linux hosts

When CONFIG_LINUX is off, devices "virtio-keyboard-device",
"virtio-mouse-device", "virtio-tablet-device" and
"virtio-input-host-device" aren't compiled in, yet
"virtio-keyboard-pci", "virtio-mouse-pci", "virtio-tablet-pci" and
"virtio-input-host-pci" still are.  Attempts to introspect them crash,
e.g.

    $ qemu-system-x86_64 -device virtio-tablet-pci,help
    **
    ERROR:/work/armbru/qemu/qom/object.c:333:object_initialize_with_type: assertion failed: (type != NULL)

Broken in commit 710e2d9 and commit 006a5ed.

Fix by compiling the "virtio-FOO-pci" exactly when compiling the
"virtio-FOO-device": compile "virtio-keyboard-device",
"virtio-mouse-device", "virtio-tablet-device" regardless of
CONFIG_LINUX, and compile "virtio-input-host-pci" only for
CONFIG_LINUX.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <1444320700-26260-3-git-send-email-armbru@redhat.com>
