build: silence cppi warning, clarify vbox headers

These files are borrowed from upstream release versions, and should
not need further edits in the context of libvirt (instead, a new
upstream vbox release would entail adding a new header file).  We do
not re-generate these files as part of libvirt, nor do we want to lose
our minor edits (such as cppi cleanups).

* src/vbox/vbox_CAPI_v2_2.h: Clarify file origins.
* src/vbox/vbox_CAPI_v3_0.h: Likewise.
* src/vbox/vbox_CAPI_v3_1.h: Likewise.
* src/vbox/vbox_CAPI_v3_2.h: Likewise.  Reindent with cppi.

src/vbox/vbox_CAPI_v2_2.h

-
+/*
+ * Libvirt notice: this file is derived from the VirtualBox SDK, with
+ * libvirt edits; do not regenerate in the context of libvirt.
+ */
 /*
  *  DO NOT EDIT! This is a generated file.
  *

src/vbox/vbox_CAPI_v3_0.h

-
+/*
+ * Libvirt notice: this file is derived from the VirtualBox SDK, with
+ * libvirt edits; do not regenerate in the context of libvirt.
+ */
 /*
  *  DO NOT EDIT! This is a generated file.
  *

src/vbox/vbox_CAPI_v3_1.h

-
+/*
+ * Libvirt notice: this file is derived from the VirtualBox SDK, with
+ * libvirt edits; do not regenerate in the context of libvirt.
+ */
 /*
  *  DO NOT EDIT! This is a generated file.
  *