In common with VNC, the QEMU driver configuration file is used
specify the host level TLS certificate location and a default
password / listen address

* src/qemu/qemu.conf: Add spice_listen, spice_tls,
  spice_tls_x509_cert_dir & spice_password config params
* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Parsing of
  spice config parameters and updating -spice arg generation
  to use them
* tests/qemuxml2argvdata/qemuxml2argv-graphics-spice-rhel6.args,
  tests/qemuxml2argvtest.c: Expand test case to cover driver
  level configuration