libvirt_qemu_probes.stp stopped working after switching to a build
that used --with-driver-modules. This was because the symbols listed
int libvirt_qemu_probes.stp are no longer in $(bindir)/libvirtd, but
are now in $(libdir)/connection-driver/libvirt_driver_qemu.so.

This patch enhances dtrace2systemtap.pl (which generates the .stp
files from .d files) to look for a new "module" setting in the
comments of the .d file (similar to the existing "binary" setting),
and to look for a --with-modules option. If the --with-modules option
is set *and* a "module" setting is present in the .d file, the process
name for the stap line is set to

   $libdir/$module

If either of these isn't true, it reverts to the old behavior.

src/Makefile.am was also modified to add the --with-modules option
when the build calls for it, and src/libvirt_qemu_probes.d has added a
"module" line pointing to the correct .so file for the qemu driver.