vbox: add support for v4.2.20+ and v4.3.4+

vbox: add support for v4.2.20+ and v4.3.4+

Bugs have been found in the VirtualBox API C bindings. These bugs have
been fixed in versions 4.2.20 and 4.3.4. However, the changes in the
C bindings are incompatible with the vbox_CAPI_v4_2.h and vbox_CAPI_v4_3.h
files which are bundled in libvirt source code.
This is why the following patch adds vbox_CAPI_v4_2_20.h and
vbox_CAPI_v4_3_4.h.

The actual underlying problem here is that until now,
libvirt assumed that VirtualBox API can only change between minor
versions (4.2 -> 4.3), but we have a case here where it changed
(or got fixed) between patch versions (4.2.18 -> 4.2.20).

This patch makes the VBOX_API_VERSION represent the full API
version number (i.e 4002 => 4002000) so there are specific version
numbers for Vbox 4.2.20 (4002020) and 4.3.4 (4003004)