Set virt_use_nfs when NFS pool is added.

selinux has a special boolean to make it easier for disk images
to be managedi by libvirt. Set this to true when a user
adds an NFS storage pool.

Most virtualzation documentation recommends that this be set
to true. For example:
http://www.ovirt.org/Troubleshooting_NFS_Storage_Issues
http://fedoraproject.org/wiki/How_to_debug_Virtualization_problems

This will leave it set to true, even if
the user removes NFS storage pools. It is not a security risk, and
we should not set it to False in case it had already been set by the
user for another non-kimchi use.

Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>