Important: libvirt security update (Oracle Linux 7)

[5.7.0-15.el7]
- qemu: Escape the qemu driver systemd DOT hoax (Wim ten Have) [Orabug:
31380815]

[5.7.0-14.el7]
- vmx: make 'fileName' optional for CD-ROMs (Pino Toscano) [Orabug: 31350200]
- vmx: shortcut earlier few 'ignore' cases in virVMXParseDisk() (Pino Toscano) [Orabug: 31350200]
- domain group: Fix a potential SEGV while restoring guest domains (Wim ten Have) [Orabug: 31285615]
- cpu_map: Distinguish Cascadelake-Server from Skylake-Server (Jiri Denemark) [Orabug: 31214897]
- cpu_map: Add more -noTSX x86 CPU models (Christian Ehrhardt) [Orabug: 31214897]
- qemuDomainGetStatsIOThread: Don't leak array with 0 iothreads (Peter Krempa) [Orabug: 31251756] {CVE-2020-12430}