tests: Make qemuAgent single sync

tests: Make qemuAgent single sync

The qemuAgent has option to issue guest-sync command before each
intended command or issue the sync commend just once, right after
the socket is opened and before the first intended command is
issued. The latter is referred to as single sync agent and is
enabled by VSERPORT_CHANGED event which allows us to detect
when the agent (dis-)connects in the guest.

Now, every QEMU that we support (4.2.0 or newer) has the event
and thus will use single sync agent. Therefore, adjust
qemuagenttest to make it test what's used in the real world,
rather than old approach.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>