qemu: split out vte into seperate PACKAGECONFIG

qemu: split out vte into seperate PACKAGECONFIG

* vte-native requires host compiler supporting c++17. Some distros we support
  ship comilers not supporting c++17 currently.
* oe-self-test 'test_testimage_virgl_gtk' sets
  PACKAGECONFIG_append_pn-qemu-system-native = " gtk+". With vte enabled by
  'gtk+'-config, build fails on hosts with ancient compilers [1].
* Alexander Kanavin did some tests: Without vte in PACKAGECONFIG there is still
  a working terminal available with reduced functionality [2].
* Users wanting 'full' terminal in system-qemu back should add
  PACKAGECONFIG_append_pn-qemu-system-native = " vte"
  in their local.conf.

The idea of this patch was coming from Ross Burton - thanks!

[1] http://lists.openembedded.org/pipermail/openembedded-core/2019-April/281637.html
[2] http://lists.openembedded.org/pipermail/openembedded-core/2019-April/281727.html

(From OE-Core rev: 78e160ac4b72bc3cfbdd89492c711a318585328b)

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>