vbox: Rename #include guard macro in header files

vbox: Rename #include guard macro in header files

To avoid including a header file more than once, either:

  #pragma once

can be used, or the older trick (that vbox still uses):

  #ifndef MACRO
  # define MACRO

Well, vbox still uses the latter and in its 7.0 release the macro
was renamed from ___VirtualBox_CXPCOM_h to ___VirtualBox_CAPI_h.
Now, ideally, we wouldn't touch those header files for older
versions, but we need to use the same macro across all header
files (because vbox_tmpl.c includes corresponding vbox_CAPI_XXX.h
and then includes vbox_XPCOMCGlue.h which in turn includes
vbox_CAPI_v6_1.h to get the basic typedefs).

Instead of changing the newer 7.0 header file (and having to
change all subsequent versions), let's change the old ones and as
we drop support for them, we can forget this ever happened.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>