[RFC] tools: don't use qemu default config

[Jim Fehlig <jfehlig@suse.com>]

I recently changed SUSE's Xen package to use the distro qemu instead of building
qemu-xen. This got some other eyes looking at Xen's use of qemu and it was
noticed that libxl and xen-qemu-dom0-disk-backend.service do not include
'-no-user-config' when invoking qemu. The latter also does not include
'-nodefaults'. Commit 6ef823fd added '-nodefaults' to the qemu args created by
libxl, but missed adding it to the qemu args in xen-qemu-dom0-disk-backend.service.

I _think_ adding '-nodefaults' to the qemu args in the service file is
non-controversial. What do folks think of also adding '-no-user-config'? It
seems the global config in /etc/qemu/qemu.conf would end up being more
problematic than helpful for Xen.

As a side note, the libvirt qemu driver includes '-no-user-config -nodefaults'
in all its qemu invocations to avoid configuration which it doesn't control.

WRT qemu args, another suggestion was to explicitly specify 'accel=xen' in the
machine arg. Together, these changes would e.g. result in the service file qemu
args changing slightly to

  -machine xenpv,accel=xen -xen-domid 0 -xen-attach -name dom0 \
  -daemonize -no-user-config -nodefaults -display none \
  -pidfile /var/run/xen/qemu-dom0.pid

If folks agree with these changes, I'll be happy to provide a patches for libxl
and the systemd service file. Thanks for your comments.

Regards,
Jim

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