Add support for enabling SASL for SPICE guests

QEMU has support for SASL auth for SPICE guests, but libvirt
has no way to enable it. Following the example from VNC where
it is globally enabled via qemu.conf

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

tests/qemuxml2argvdata/qemuxml2argv-graphics-spice-sasl.args

@@ -0,0 +1,9 @@
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test \
+SASL_CONF_PATH=/root/.sasl2 QEMU_AUDIO_DRV=spice \
+/usr/bin/qemu -S -M pc -m 214 -smp 1 -nodefaults \
+-monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb -hda \
+/dev/HostVG/QEMUGuest1 \
+-spice port=5903,tls-port=5904,sasl,addr=127.0.0.1,\
+x509-dir=/etc/pki/libvirt-spice,tls-channel=default \
+-vga qxl -global qxl.ram_size=67108864 -global \
+qxl.vram_size=18874368 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

tests/qemuxml2argvdata/qemuxml2argv-graphics-spice-sasl.xml

@@ -0,0 +1,35 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219100</memory>
+  <currentMemory unit='KiB'>219100</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <driver name='qemu' type='raw'/>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
+    </disk>
+    <controller type='usb' index='0'/>
+    <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <input type='mouse' bus='ps2'/>
+    <graphics type='spice' port='5903' tlsPort='5904' autoport='no' listen='127.0.0.1' defaultMode='secure'>
+      <listen type='address' address='127.0.0.1'/>
+    </graphics>
+    <video>
+      <model type='qxl' ram='65536' vram='18432' heads='1'/>
+    </video>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>