qemuDomainDefPostParse: Fetch qemuCaps from domain object
    
    We can't rely on def->emulator path. It may be provided by user
    as we give them opportunity to provide their own XML for
    migration. Therefore the path may point to just whatever binary
    (or even to a non-existent file). Moreover, this path is meant
    for destination, but the capabilities lookup is done on source.
    What we can do is to assume same capabilities for post parse
    callbacks as the running domain has. They will be used just to
    add some default models/controllers/devices/... anyway.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>