Use name instead of pid in QEMU logs.

Use name instead of pid in QEMU logs.

Change the logfile name generated by qemu-dm to use the pattern
qemu-dm-[NAME].log instead of qemu-dm-[PID].log. This makes it
easier for the adminsitrator to figure out which log corresponds to
which guest (particularly after a crash where you no long know what
PID the qemu-dm process for your guest had). It also prevents the
number of log files from growing unbounded.

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