virCapabilitiesDomainDataLookup: Produce saner error message

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>