block: Add VFIO based NVMe driver

This is a new protocol driver that exclusively opens a host NVMe
controller through VFIO. It achieves better latency than linux-aio by
completely bypassing host kernel vfs/block layer.

    $rw-$bs-$iodepth  linux-aio     nvme://
    ----------------------------------------
    randread-4k-1     10.5k         21.6k
    randread-512k-1   745           1591
    randwrite-4k-1    30.7k         37.0k
    randwrite-512k-1  1945          1980

    (unit: IOPS)

The driver also integrates with the polling mechanism of iothread.

This patch is co-authored by Paolo and me.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20180116060901.17413-4-famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>