Autoconf: add variable for pass arbitrary options to qemu upstream

Added configure options for pass arbitrary configure options to qemu
upstream build.

Usage example:
./configure
--with-extra-qemuu-configure-args="--enable-spice --enable-usb-redir"

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>

---

Changes in v7:
- Changed from AC_ARG_VAR to AC_ARG_WITH and other small changes following
  Ian Campbell advices

Changes in v6:
- redid the patch similar to v1 adding variable for pass arbitrary
  options to qemu upstream instead specific spice and usbredir ones