accel: Rename 'init' method to 'init_machine'

accel: Rename 'init' method to 'init_machine'

Today, all accelerator init functions affect some global state:
* tcg_init() calls tcg_exec_init() and affects globals such as tcg_tcx,
  page size globals, and possibly others;
* kvm_init() changes the kvm_state global, cpu_interrupt_handler, and possibly
  others;
* xen_init() changes the xen_xc global, and registers a change state handler.

With the new accelerator QOM classes, initialization may now be split in two
steps:
* instance_init() will do basic initialization that doesn't affect any global
  state and don't need MachineState or MachineClass data. This will allow
  probing code to safely create multiple accelerator objects on the fly just
  for reporting host/accelerator capabilities, for example.
* accel_init_machine()/init_machine() will save the accelerator object in
  MachineState, and do initialization steps which still affect global state,
  machine state, or that need data from MachineClass or MachineState.

To clarify the difference between those two steps, rename init() to
init_machine().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>