arm: multi_v7_defconfig: Add virtio drivers

To make multi_v7_defconfig really "multi", let's add virtio drivers
to allow booting a default kernel as a KVM/QEMU guest.
MACH_VIRT is already defined, so we just add VIRTIO_BLK, VIRTIO_NET
and VIRTIO_CONSOLE to actually make this kernel useful in a guest.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>