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>

MAINTAINERS

@@ -1888,6 +1888,12 @@ L: qemu-block@nongnu.org
 S: Supported
 F: block/null.c
 
+NVMe Block Driver
+M: Fam Zheng <famz@redhat.com>
+L: qemu-block@nongnu.org
+S: Supported
+F: block/nvme*
+
 Bootdevice
 M: Gonglei <arei.gonglei@huawei.com>
 S: Maintained

block/Makefile.objs

@@ -11,6 +11,7 @@ block-obj-$(CONFIG_POSIX) += file-posix.o
 block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
 block-obj-y += null.o mirror.o commit.o io.o
 block-obj-y += throttle-groups.o
+block-obj-$(CONFIG_LINUX) += nvme.o
 
 block-obj-y += nbd.o nbd-client.o sheepdog.o
 block-obj-$(CONFIG_LIBISCSI) += iscsi.o