util: Allow port allocator to skip bind() check
    
    Test suites using the port allocator don't want to have different
    behaviour depending on whether a port is in use on the host. Add
    a VIR_PORT_ALLOCATOR_SKIP_BIND_CHECK which test suites can use
    to skip the bind() test. The port allocator will thus only track
    ports in use by the test suite process itself. This is fine when
    using the port allocator to generate guest configs which won't
    actually be launched
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>