libvirt: Introduce virDomainGraphicsReload API

libvirt: Introduce virDomainGraphicsReload API

The new virDomainGraphicsReload API is used to make the domain reload
its certificates without restart, and avoid service interruption.

Currently, only QEMU VNC TLS certificates are supported, but
flags are also reserved for subsequent scenarios.

To reload QEMU VNC TLS certificates as an example, we can call:

  virDomainGraphicsReload(domain, 0, 0);

Then the specified QMP message would be send to QEMU:
{"execute": "display-reload", "arguments":{"type": "vnc", "tls-certs": true}}

Signed-off-by: Zheng Yan <yanzheng759@huawei.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>