qemu_agent: add qemuAgentGetDisks

qemu_agent: add qemuAgentGetDisks

guest-get-disks is available since QEMU 5.2:
https://wiki.qemu.org/ChangeLog/5.2#Guest_agent

Note that the test response was manually edited based on a reply on my
bare-metal computer. It shows partial results due to pcieport driver not
being currently supported by QGA.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Tested-by: Han Han <hhan@redhat.com>