Autoconf: add options to enable spice and usbredir in qemu upstream

Added configure options to enable spice and usbredir to upstream
qemu build done with xen.

Note: I wait feedback and I'll improve it if needed.

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

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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