Bindings for libvirt

Safe and idiomatic Rust bindings to libvirt.

For the automatically generated low level FFI bindings, see the virt-sys crate in this repo.

Prior/related work

There are existing bindings to libvirt available. Libvirt hosts what seems to be the official bindings at https://libvirt.org/git/?p=libvirt-rust.git;a=summary. I tried to use those bingings at first, but ran into too many obstacles. I then attempted to contribute some patches, but using a mail list for this is just not going to cut it for me. There is no way I will expose SMTP to my dev machine, and I did not manage to send a proper thread from my email client.

The issues I had with the official bindings:

• Explicitly states that they are a direct mapping of the underlying C API. Meaning it intentionally won't do proper resource freeing on dropped instances and other things one would expect from a nice Rust API.
• Unsoundness/invalid memory access in Stream::recv. https://www.redhat.com/archives/libvir-list/2019-September/msg00764.html
• Converts between C stings and Rust strings without any error handling. Panics on any string content that is not compatible in the other format.
• Unsoundness in all constructors. All safe wrappers can be created without unsafe with raw pointers. Meaning any subsequent use will cause undefined behavior:

  // This will segfault
  virt::connect::Connect::new(ptr::null_mut()).get_hostname()

  These constructons must be unsafe. They should probably be named from_ptr and be put further down in the impl block as well. They are not supposed to be the main way of creating the safe wrappers. https://www.redhat.com/archives/libvir-list/2019-September/msg01139.html
• No separation between raw FFI bindings and higher level safe bindings. The more standard design would be to have a separate virt-sys crate with bindgen generated direct bindings. This will make maintaining and verifying the raw bindings easier, and the source files of the higer level crate don't get cluttered with thousands of lines of extern declarations. It also directly exposes the entire surface area of libvirt. So even if ideal usage of the bindings never need to go down to this layer, it is possible, if the safe wrappers are missing some feature or it's implemented in a way that is not suitable for the current use case. Even if my bindings and the official ones were to remain separate projects forever, it would be beneficial for everyone if we shared the same -sys crate with low level bindings.
• Very nitpicky, but the code style is very C-isch instead of Rust. Explicit return statements where implicitly returning would work fine etc.

The best would of course be if there were one set of awesome bindings. So if you work on the official bindings and would like to collaborate, let's do that!