domcapabilities: fix CPUModel usable attribute

It is not True/False attribute, it can contain 'unknown' value as well
which means libvirt cannot figure out whether the CPU model is usable
with that specific QEMU binary.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>

virtinst/domain/cpu.py

@@ -239,7 +239,7 @@ class DomainCpu(XMLBuilder):
             return
 
         cpu_model = domcaps_mode.get_model(self.model)
-        if cpu_model and cpu_model.usable:
+        if cpu_model and cpu_model.usable != "no":
             return
 
         logging.debug("Host capabilities CPU '%s' is not supported "

virtinst/domcapabilities.py

@@ -94,7 +94,7 @@ class _Features(_CapsBlock):
 class _CPUModel(XMLBuilder):
     XML_NAME = "model"
     model = XMLProperty(".")
-    usable = XMLProperty("./@usable", is_yesno=True)
+    usable = XMLProperty("./@usable")
     fallback = XMLProperty("./@fallback")