ioemu: Send logs to stderr and have xend redirect stderr to the

ioemu: Send logs to stderr and have xend redirect stderr to the
correct log file.

At the same time, this patch renames the logfile to be
'qemu-dm-{NAME}.log' instead of qemu-dm-{ID}.log.  This makes it
easier to track/find the QEMU logfile associated with a VM. It will
also save 1 backup qemu-dm-{NAME}.log.1 so if a domain crashes &
restart, you don't loose/overwrite the logfile immediately.

Finally it changes the QEMU monitor prompt back to '(qemu)' instead of
'(HVMXen)' because automated tools /scripts interacting with QEMU's
monitor need a consistent prompt to look for & changing it for Xen
serves no useful purpose.

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