Dispatch hooks use "vp" rather than "vm"

Dispatch hooks use "vp" rather than "vm"

* libguile/vm.c (vm_dispatch_hook):
  (vm_dispatch_apply_hook):
  (vm_dispatch_push_continuation_hook):
  (vm_dispatch_pop_continuation_hook):
  (vm_dispatch_next_hook):
  (vm_dispatch_abort_hook):
  (vm_dispatch_restore_continuation_hook): Use scm_vm pointer instead of
  Scheme vm object.