qemu: add helper for getting guest users

qemu: add helper for getting guest users

This function fetches the list of logged-in users from the qemu agent
and adds them to a list of typed parameters so that they can be used
internally in libvirt.

Also add some basic tests for the function.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>