qemu: include hostname in QEMU log files

Often when debugging bug reports one is given a copy of the file
from /var/log/libvirt/qemu/$NAME.log along with other supporting
files. In a number of cases I've been given sets of files which
were from different machines. Including the hostname in the QEMU
log file will help identify when the bug reporter is providing
bad information.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>