[libvirt] QEmu: support disk filenames with comma

Hi! This patch fixes RHBZ #801036 adding support to filenames with comma using QEmu. As this is my first patch to libvirt, I'm not completely sure if I should have also changed other parts of the code, or if I have used the wrong memory allocation functions/macros, or if I shouldn't have created a separate commit to add my name to the AUTHORS file... :-) I also need to make it clear that this patch only fixes the problem in the "libvirt data -> QEmu args" way (e.g., when QEmu reads an XML file to start QEmu, or when using virsh domxml-to-native). The other way, "QEmu args -> libvirt data" is *not* implemented (e.g., virsh domxml-from-native). I would like to know if I'm going in the right way so I can continue working on this patch, and if that half of the feature is also needed. Any feedback is welcome, thanks! Best regards, Crístian.