Autoconf: add variable for pass arbitrary options to qemu upstream

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

Usage example:
./configure QEMUU_EXTRA_ARGS="--enable-spice --enable-usb-redir"

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

---

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

Changes in v5:
- improved README changes
- changed qemuu_add_par variable to qemuu_extra_args
- fixed some indentation errors
- changed 2 additions that was bash-only
- run autoconf to update configure file

Changes in v4:
- removed upstream qemu debug option already included by default if
  tools debug is enabled.
- refresh for latest xen unstable git