hw/virtio: enable common virtio feature for mmio device

hw/virtio: enable common virtio feature for mmio device

Both 'indirect_desc' and 'event_idx' are bus independent features,
and they should be enabled for mmio devices too.

On arm64 quad core VM(qemu-kvm), the patch can increase block I/O
performance a lot with latest linux tree:
        - without the patch: 14K IOPS
        - with the patch: 34K IOPS

fio script:
        [global]
        direct=1
        bsrange=4k-4k
        timeout=10
        numjobs=4
        ioengine=libaio
        iodepth=64

        filename=/dev/vdc
        group_reporting=1

        [f1]
        rw=randread

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>