[libvirt] Patch to allow setting of svirt XML.

This patch fixes the seclabel handling in domain_conf.c to allow virt-manager to set the seclabel model, type and label. Also adds missing error messages when the xml is incorrect. How much verification should we be doing on this? I have another patch that verifies the model as being a known model and a patch to verify the label is a correct label. (IE SELinux verifies the label is understood by the kernel.) The problem with this second patch is it sucks in security.[ch], security_selinux.[ch] into the libvirt_lxc. Should I be doing this?