maint: don't distribute generated .def files

maint: don't distribute generated .def files

I ran 'make dist' in the directory left over from ./autobuild.sh
(which was configured for a mingw cross build); the resulting
tarball had more files than 'make dist' on a normal Linux build.
I traced it to the fact that we were distributing a generated
file, but only when configure said the end user had to generate
the file in the first place.  In the process, I noticed that
we had some difference in symbol file names; I added a comment
explaining why the difference exists (after first trying to
normalize the names and hitting VPATH build failures).

* configure.ac (LIBVIRT_QEMU_SYMBOL_FILE): Add some comments.
* src/Makefile.am (EXTRA_DIST): No need to ship a generated file;
particularly since which file is built depends on configure results.