vmspawn-varlink: drop AcquireQMP stub and QemuMachineInstance interface

vmspawn-varlink: drop AcquireQMP stub and QemuMachineInstance interface

The AcquireQMP() method was a placeholder that always returned
EOPNOTSUPP, reserving room for a future id-rewriting QMP multiplex
proxy. The broader direction is for systemd-vmspawn to remain the single
source of truth for VM control rather than exposing raw QMP to clients.

Since AcquireQMP was the only method on io.systemd.QemuMachineInstance
(and AlreadyAcquired was its only error), remove the whole interface
along with the stub, and update the controlAddress field comment in
io.systemd.Machine to stop referencing it.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>