build: Ensure all spec files have mandatory %files list

A spec file must have a %files list, even if the list is empty.  Some of
our base packages don't have %files lists, which causes mock to build
them and throw them away.   We still generate Makefile rules to build
these files, so they are rebuilt and thrown away every time we run make.

For library packages, we should move the libraries into the base package;
keep only the headers, interfaces or other developer-focussed files
in the -devel package, and have the -devel package depend on the base
package.

Even with this change there is still some overbuilding, which seems to be
caused by the libvirt-daemon-driver-uml, libvirt-daemon-qemu and libvirt-daemon-uml
targets in the makefile.  We generate make targets for these, but we don't 
build (or at least keep) the packages - presumably they are not built because
of conditionals in the spec file.   It's annoying that rpmlib seems to be giving us
the impression that these are real packages that will be built - I thought it was 
correctly handling all the conditional definitions.