virCapabilitiesDomainDataLookup: Produce saner error message
    
    During a review, I've noticed this error message that was eventually
    produced when I was trying to define a domain:
    
    error: invalid argument: could not find capabilities for arch=mips64el
    domaintype=(null)
    
    Look at the (null). Why is it there? Well, during XML parsing, we try
    to look up the default emulator for given OS type and possibly virt
    type too. And this is the problem, because if we don't want to look up
    by virt type, a -1 is passed to note this fact. Later, the code
    handles -1 just right. Except for error message. When it is
    constructed (in a very fabulous way I must say), the value is compared
    to zero, not -1. And since we don't have any translation from -1 to a
    virt type string, we just print (null).
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>