xml: omit domain name from comment if it contains double hyphen
xml: omit domain name from comment if it contains double hyphen

https://bugzilla.redhat.com/show_bug.cgi?id=868692

We put a comment containing "virsh edit <domain_name>" at the start of
the XML. W3C recommendation forbids the use of "--" in comments [1] and
libvirt can't parse it either. This patch omits the domain name if it
contains a double hyphen.

[1] http://www.w3.org/TR/REC-xml/#sec-comments
(cherry picked from commit 9b704ab8235af010b1fda4886201aab02098b969)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2