Add support for libvirt 5.7.0 and new features from 6.x and 7.x

This commit introduces a pattern by which libvirt-rust can be compatible with a range of libvirt versions.

To support libvirt as old as 5.7.0, one can do:

[dependencies]
virt = { version = "0.2", default-features = false }

To enable bindings which require a newer libvirt than supported in the default feature set, one can do:

[dependencies]
virt = { version = "0.2", features = ["libvirt-6-10-0"] }

The default build (virt = "0.2") remains compatible with libvirt 6.0.0, same as before this commit.