vmspawn: convert network device setup to bridge

vmspawn: convert network device setup to bridge

Remove the static netdev/nic INI config sections for both privileged
TAP, nsresourced TAP, and user-mode networking. Replace them with a
NetworkInfo struct that captures the network type, TAP fd or interface
name, and MAC address, passed to vmspawn_varlink_setup_network() for
runtime configuration via QMP.

For the nsresourced TAP path the fd is now passed to QEMU via QMP
getfd + SCM_RIGHTS instead of being inherited through pass_fds.

Declare the MachineConfig aggregate that this and the following
conversion patches populate, zero-initialized with explicit -EBADF
for the fd fields so every sub-structure cleans up safely regardless
of which device types the invocation ends up using.

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