vl: move all generic initialization out of vl.c

qdev_machine_creation_done is only setting a flag now.  Extend it to
move more code out of vl.c.  Leave only consistency checks and gdbserver
processing in qemu_machine_creation_done.

gdbserver_start can be moved after qdev_machine_creation_done because
it only does listen on the socket and creates some internal data
structures; it does not send any data (e.g. guest state) over the socket.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>