Add SPICE support for QEMU driver configuration file

Add SPICE support for QEMU driver configuration file

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