qemu: Use domid in qemu log name. Can be related back to domain name

qemu: Use domid in qemu log name. Can be related back to domain name
or uuid via xend logs, less unique than pids to prevent excessive
number of log files hanging around, yet not immediately reused (unlike
e.g., after domain crash or reboot) where previous log file is
immediately overwritten. Not using domain names in logs avoids and
chance of unprintable charcatres becoming embedded in filenames.
Signed-off-by: Keir Fraser <keir@xensource.com>