enhance hostdev mode 'capabilities' process
    
    Currently, only LXC has hostdev mode 'capabilities' support,
    so the other drivers should forbid to define it in XML.
    The hostdev mode check is added to devicesPostParseCallback()
    for each hypervisor driver.
    
    But there are some drivers lack function devicesPostParseCallback(),
    so only add check for qemu, libxl, openvz, uml, xen, xenapi.
    
    Signed-off-by: Jincheng Miao <jmiao@redhat.com>