qemu-options.def: add to generated header list

All files include qemu-options.h which pulls in qemu-options.def from
the root directory.  Thus generating qemu-options.def from Makefile.objs
under the target directory is not effective.

Further, people expect .def file to get cleaned with make clean:
it does not have state so no reason to defer removing it
until distclean. Also add a rule to remove old files that might
be around.

This fixes the error: ‘QEMU_OPTION_spice’ undeclared
(first use in this function) error that some people reported
which is really down to an out of date .def file.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>