fence-virtd: Allow multiple hypervisors for the libvirt backend

Allow users to specify multiple hypervisor URIs for the
libvirt backend.

.e.g,

backends {
	libvirt {
		uri = "qemu:///system";
		uri1 = "qemu:///system2";
		uri2 = "qemu:///system3";
	}
}

Resolves: rhbz#914144

Signed-off-by: Ryan McCabe <rmccabe@redhat.com>