build: silence cppi warning, clarify vbox headers

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.