crypto: ensure we use a predictable TLS priority setting
    
    The TLS test cert generation relies on a fixed set of algorithms that are
    only usable under GNUTLS' default priority setting. When building QEMU
    with a custom distro specific priority setting, this can cause the TLS
    tests to fail. By forcing the tests to always use "NORMAL" priority we
    can make them more robust.
    
    Reviewed-by: Eric Blake <eblake@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
    (cherry picked from commit 057ad0b46992e3ec4ce29b9103162aa3c683f347)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>