Libtool supports linking directly against .o files on some platforms
(such as Linux), which happens to be the only place where we are
actually doing that (for the dtrace-generated probes.o files).  However,
it raises a big stink about the non-portability, even though we don't
attempt it on platforms where it would actually fail:

  CCLD   libvirt_driver_qemu.la

*** Warning: Linking the shared library libvirt_driver_qemu.la against
the non-libtool
*** objects  libvirt_qemu_probes.o is not portable!

This shuts libtool up by creating a proper .lo file that matches
what libtool normally expects.

* src/Makefile.am (%_probes.lo): New rule.
(libvirt_probes.stp, libvirt_qemu_probes.stp): Simplify into...
(%_probes.stp): ...shorter rule.
(CLEANFILES): Clean new .lo files.
(libvirt_la_BUILT_LIBADD, libvirt_driver_qemu_la_LIBADD)
(libvirt_lxc_LDADD, virt_aa_helper_LDADD): Link against .lo file.
* tests/Makefile.am (PROBES_O, qemu_LDADDS): Likewise.