workaround against libvirt image permission denied

On Fedora, when KVM images don't reside in /var/lib/libvirt/images, virsh failed to create such a vm and complains
about as follows:

[root@fedora17 winxp]# virsh create winxp.xml
error: Failed to create domain from winxp.xml
error: internal error Process exited while reading console log output: char device redirected to /dev/pts/3
do_spice_init: starting 0.10.1
spice_server_add_interface: SPICE_INTERFACE_MIGRATION
spice_server_add_interface: SPICE_INTERFACE_KEYBOARD
spice_server_add_interface: SPICE_INTERFACE_MOUSE
qemu-kvm: -drive file=/var/lib/libvirt/images/winxp/winxp.img,if=none,id=drive-virtio-disk0,format=qcow2,cache=directsync: could not open disk image /var/lib/libvirt/images/winxp/winxp.img: Permission denied

To get rid of such a restriction,

# vi /etc/libvirt/qemu.conf
...
user = "root"
group = "root"
dynamic_ownership = 0
...