[Libguestfs] [PATCH nbdkit] Add support for AF_VSOCK.

This is a series of patches to libnbd and nbdkit adding AF_VSOCK support. On the host side it allows you to start an nbdkit instance which listens on a virtio-vsock socket: $ ./nbdkit -fv --vsock memory 1G ... nbdkit: debug: bound to vsock 2:10809 On the guest side you can then use libnbd to connect to the server: $ ./run nbdsh -c 'h.connect_vsock(2, 10809)' -c 'print(h.get_size())' 1073741824 $ ./run nbdfuse mp --vsock 2 10809 & $ ll mp/ total 0 -rw-rw-rw-. 1 rjones rjones 1073741824 Oct 18 16:23 nbd $ dd if=/dev/random of=mp/nbd bs=1024 count=100 conv=notrunc,nocreat dd: warning: partial read (84 bytes); suggest iflag=fullblock 0+100 records in 0+100 records out 6851 bytes (6.9 kB, 6.7 KiB) copied, 0.013797 s, 497 kB/s (Performance of FUSE is not great, it should be better using raw libnbd.) I mainly wrote this to show that it can be done. It's unclear if this would be faster or slower than the usual way that NBD devices are exposed to guests via virtio-blk/-scsi. https://wiki.qemu.org/Features/VirtioVsock Thanks: Stefan Hajnoczi for help with debugging this.