build: Update govirt and rest requirements
    
    With these new values, 0.3.3 for libgovirt and 0.8 for librest, we can
    remove checks for OVIRT_REST_CALL_ERROR_CANCELLED and correspondent
    rest_proxy_auth_cancel().
    
    Distros that already ship these versions, such as Fedora, RHEL 7.4
    onwards, and Ubuntu since 17.10.
    
    Acked-by: Christophe Fergeau <cfergeau@redhat.com>
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>