vmspawn: QMP-varlink bridge for VM runtime control

vmspawn: QMP-varlink bridge for VM runtime control

Create a QMP socketpair for QEMU machine monitor control, configure
the QMP chardev+mon via the QEMU config file, and wire up the bridge
infrastructure.

After fork, vmspawn initializes the QMP bridge, probes QEMU feature
support synchronously (driving the QMP handshake to RUNNING
transparently), resumes vCPUs, then sets up the varlink server for
runtime VM control. The control socket path is passed to machined via
the controlAddress field in machine registration.

Device configuration still uses the legacy INI config path and will
be converted to bridge calls in subsequent commits.

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