docs: Add section for NVMe VFIO driver

With sufficiently new versions of libssh2 and OpenSSH, @code{fsync} is

supported.

@node disk_images_nvme

@subsection NVMe disk images

NVM Express (NVMe) storage controllers can be accessed directly by a userspace

driver in QEMU.  This bypasses the host kernel file system and block layers

while retaining QEMU block layer functionalities, such as block jobs, I/O

throttling, image formats, etc.  Disk I/O performance is typically higher than

with @code{-drive file=/dev/sda} using either thread pool or linux-aio.

The controller will be exclusively used by the QEMU process once started. To be

able to share storage between multiple VMs and other applications on the host,

please use the file based protocols.

Before starting QEMU, bind the host NVMe controller to the host vfio-pci

driver.  For example:

@example

# modprobe vfio-pci

# lspci -n -s 0000:06:0d.0

06:0d.0 0401: 1102:0002 (rev 08)

# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind

# echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id

# qemu-system-x86_64 -drive file=nvme://@var{host}:@var{bus}:@var{slot}.@var{func}/@var{namespace}

@end example

Alternative syntax using properties:

@example

qemu-system-x86_64 -drive file.driver=nvme,file.device=@var{host}:@var{bus}:@var{slot}.@var{func},file.namespace=@var{namespace}

@end example

@var{host}:@var{bus}:@var{slot}.@var{func} is the NVMe controller's PCI device

address on the host.

@var{namespace} is the NVMe namespace number, starting from 1.

@node disk_image_locking

@subsection Disk image file locking

* disk_images_iscsi::         iSCSI LUNs

* disk_images_gluster::       GlusterFS disk images

* disk_images_ssh::           Secure Shell (ssh) disk images

* disk_images_nvme::          NVMe userspace driver

* disk_image_locking::        Disk image file locking

@end menu