qemu: arm: Initialise virtio devices in board_late_init

On the qemu arm platform, the virtio devices are initialised in the
board_init function, which gets called before the initr_pci. With
this sequence, the virtio block devices on the pci bus are not
initialised. Move the initialisation of the virtio devices to
board_late_init which gets called after the call to initr_pci.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>