tools: Restrict configuration of qemu processes

tools: Restrict configuration of qemu processes

Commit 6ef823fd added '-nodefaults' to the qemu args created by
libxl, which is a good step in restricting qemu's default
configuration. This change takes another step by adding
-no-user-config, which ignores any user-provided config files in
sysconfdir. Together, -nodefaults and -no-user-config allow Xen
to avoid unkown and uncontrolled qemu configuration.

Both options are also added to the qemu invocation in the
xen-qemu-dom0-disk-backend systemd service file.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>