qemu_capabilities: Introduce virQEMUCapsCacheLookupDefault
    
    virConnectGetDomainCapabilities needs to lookup QEMU capabilities
    matching a specified binary, architecture, virt type, and machine type
    while using default values when any of the parameters are not provided
    by the user. Let's extract the lookup code into
    virQEMUCapsCacheLookupDefault to make it reusable.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>