build: silence libtool warning on probes.o

build: silence libtool warning on probes.o

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.