Canonicalize distro & osfamily params in os_cls()

If the distro name as returned by libguestfs's inspect_get_distro was
redhat-based, since hyphens are not allowed in module names, importing
module redhat-based would create an ImportError exception, even if file
redhat-based.py was present. Although we don't have any redhat-based.py
module and the code will still correctly fall back to importing
linux.py this can be considered as a bug. In order to overcome this, we
replace the hyphens with underscores in the distro and osfamily names.
In the future, if we have to create a module for redhat-based distros we
will name it redhat_based.py.

image_creator/os_type/__init__.py

@@ -46,17 +46,27 @@ from functools import wraps
 
 
 def os_cls(distro, osfamily):
-    """Given the distro name and the osfamily, return the appropriate class"""
-    module = None
-    classname = None
+    """Given the distro name and the osfamily, return the appropriate OSBase
+    derived class
+    """
+
+    # hyphens are not allowed in module names
+    canonicalize = lambda x: x.replace('-', '_').lower()
+
+    distro = canonicalize(distro)
+    osfamily = canonicalize(osfamily)
+
     try:
         module = __import__("image_creator.os_type.%s" % distro,
                             fromlist=['image_creator.os_type'])
         classname = distro.capitalize()
     except ImportError:
-        module = __import__("image_creator.os_type.%s" % osfamily,
-                            fromlist=['image_creator.os_type'])
-        classname = osfamily.capitalize()
+        try:
+            module = __import__("image_creator.os_type.%s" % osfamily,
+                                fromlist=['image_creator.os_type'])
+            classname = osfamily.capitalize()
+        except ImportError:
+            raise FatalError("Unknown OS name: `%s'" % osfamily)
 
     return getattr(module, classname)