capabilities: Handle missing cpu_map.xml

It shouldn't be a fatal issue. Hit on freebsd where they install
libvirt bits into /usr/local

https://www.redhat.com/archives/virt-tools-list/2015-September/msg00056.html

virtinst/capabilities.py

@@ -19,6 +19,8 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 # MA 02110-1301 USA.
 
+import logging
+import os
 import re
 
 from .cpu import CPU as DomainCPU
@@ -55,7 +57,12 @@ class _CPUMapFileValues(XMLBuilder):
     _cpu_filename = "/usr/share/libvirt/cpu_map.xml"
 
     def __init__(self, conn):
+        if os.path.exists(self._cpu_filename):
             xml = file(self._cpu_filename).read()
+        else:
+            xml = None
+            logging.debug("CPU map file not found: %s", self._cpu_filename)
+
         XMLBuilder.__init__(self, conn, parsexml=xml)
 
         self._archmap = {}