As well as taint warnings going to the main libvirt log,
add taint warnings to the per-domain logfile

  Domain id=3 is tainted: high-privileges
  Domain id=3 is tainted: disk-probing
  Domain id=3 is tainted: shell-scripts
  Domain id=3 is tainted: custom-monitor

* src/qemu/qemu_domain.c, src/qemu/qemu_domain.h: Enhance
  qemuDomainTaint to also log to the domain logfile
* src/qemu/qemu_driver.c: Pass -1 for logFD to taint methods to
  auto-append to logfile
* src/qemu/qemu_process.c: Pass open logFD at startup for taint
  methods