qemu, qemu_v8: prefer gnome-terminal over xterm when available

gnome-terminal comes by default with Ubuntu and is much more advanced
than xterm, so use it when available.
While we're at it, move common definitions to common.mk.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed by: Victor Chong <victor.chong@linaro.org>