Add qemu commandline environment variable support. (#961)

Make it easier to tweak some qemu options by allowing passthru of
command line environment variables.

- Also cleans up weird variable indirection used for qemu commandline args
  variable through `qargs` in various constructors.
- Addresses some functionality discussed in #776.

spec/unit/templates/domain_all_settings.xml

@@ -143,5 +143,9 @@
   <qemu:commandline>
     <qemu:arg value='-device'/>
     <qemu:arg value='dummy-device'/>
+    <qemu:env name='QEMU_AUDIO_DRV' value='pa'/>
+    <qemu:env name='QEMU_AUDIO_TIMER_PERIOD' value='150'/>
+    <qemu:env name='QEMU_PA_SAMPLES' value='1024'/>
+    <qemu:env name='QEMU_PA_SERVER' value='/run/user/1000/pulse/native'/>
   </qemu:commandline>
 </domain>

spec/unit/templates/domain_spec.rb

@@ -11,7 +11,6 @@ describe 'templates/domain' do
 
     def finalize!
       super
-      @qargs = @qemu_args
     end
   end
 
@@ -76,6 +75,12 @@ describe 'templates/domain' do
 
       domain.qemuargs(value: '-device')
       domain.qemuargs(value: 'dummy-device')
+
+      domain.qemuenv(QEMU_AUDIO_DRV: 'pa')
+      domain.qemuenv(QEMU_AUDIO_TIMER_PERIOD: '150')
+      domain.qemuenv(QEMU_PA_SAMPLES: '1024')
+      domain.qemuenv(QEMU_PA_SERVER: '/run/user/1000/pulse/native')
+
       domain.shares = '1024'
       domain.cpuset = '1-4,^3,6'
       domain.nodeset = '1-4,^3,6'