[Libguestfs] [PATCH nbdkit] Add ssh plugin using libssh.

This adds a simple plugin using libssh (not libssh2). The intended use for this is with virt-v2v when sourcing guests from VMware over SSH. We've had several years of problems getting our libssh-based driver into qemu. By putting it into nbdkit instead we can bypass that. However this also lets us combine ssh access with filters, in particular the recently written ‘rate’ filter. Rich.