Like SLE12 SP1, update libvirt 1.2.18 to latest stable release.

- Update to libvirt 1.2.18.1 stable release
- Inherit many upstream bug fixes
- Drop patches included in the stable release
15120b8c-libxl-no-resume-on-suspend-fail.patch,
269d39af-storage-allocation-fix.patch,
26c5fa3a-virt-aa-helper-missing-doc.patch,
2f01cfdf-virt-aa-helper-allow-ovmf.patch,
3468542f-virFileUnlink.patch,
44a54eb0-libxl-fix-refcnt-MigrationDstArgs.patch,
52970dec-virt-aa-helper-improve-valid-path.patch,
56945e13-libxl-AttachDeviceConfig-hostdev.patch,
60acb38-revert-curmem-inactive-dom.patch,
8b1d84e6-refreshVol-failure.patch,
91fdcefa-virt-aa-helper-allow-nvram.patch,
d25a5e08-virt-aa-helper-simplify-restriction-logic.patch
e0025d29-storage-mode-check.patch,
e80b84a7-libxl-acquire-job-on-migrate.patch
- Drop signature file. Stable releases are not signed