Add qemu_agent_command and example/agent.rs for it
Adds a wrapper qemu_agent_command around virDomainQemuAgentCommand, and an example for it.
Tested on my server, for a vm without guest agent:
$ cargo build --features qemu --example agent && sudo ./target/debug/examples/agent 'qemu:///system' uli-control-node-1
Finished dev [unoptimized + debuginfo] target(s) in 0.01s
libvirt: QEMU Driver error : Guest agent is not responding: QEMU guest agent is not connected
Error: error: Guest agent is not responding: QEMU guest agent is not connected [code=AgentUnresponsive (86), domain=Qemu (10)]
For a vm with guest agent running:
$ cargo build --features qemu --example agent && sudo ./target/debug/examples/agent 'qemu:///system' uli-compute-node-1
Finished dev [unoptimized + debuginfo] target(s) in 0.28s
Result: {"return":{"version":"8.0.4","supported_commands":[...]}}
First time contributing, happy to change things to meet library standards.