Forbid use of ':' in RBD pool names

Forbid use of ':' in RBD pool names

The QEMU command line syntax for RBD disks is

   file=rbd:pool/image:opt1=val1:opt2=val2...

There is no way to escape the ':' if it appears in the
pool or image name. Thus it must be explicitly forbidden
if it occurs in the libvirt XML. People are known to
be abusing the lack of escaping in current libvirt to
pass arbitrary args to QEMU.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>