m68k: virt: Kconfig minor fixes

Select VIRTIO_MENU as it is needed by VIRTIO_MMIO.

Add an ending period at the end of the virt machine help message.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220531174514.1586248-1-laurent@vivier.eu


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>