gtk: Support GTK without VTE

GTK without VTE is needed for hosts which don't support VTE (for example
all variants of MinGW), but it can also be reasonable for other hosts.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Merge remote-tracking branch 'remotes/weil/qemu-2.0' into staging

* remotes/weil/qemu-2.0:
  gtk: Support GTK without VTE

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>