tools: Make qemu-xen-traditional build optional.

tools: Make qemu-xen-traditional build optional.

Now that we have upstream qemu people may want to avoid building this extra
code.

There is a little bit of trickery in stubdom/configure.ac to ensure that the
ioemu stubdom is only built if qemu-traditional is enabled.

libxl will return an error if a caller tries to build a domain using
qemu-xen-traditional when this support was disabled at build time. Since
qemu-xen-traditional has been historically tightly bound to the Xen releases I
don't see any value in supporting "3rd party" provision of
qemu-xen-traditional.

We also do not want/need this on ARM therefore default is on for x86 and off
otherwise.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- trivial conflicts in Tools.mk.in and tools/configure.ac.
         Reran autogen.sh ]