virCaps: get rid of defaultConsoleTargetType callback

This patch refactors various places to allow removing of the
defaultConsoleTargetType callback from the virCaps structure.

A new console character device target type is introduced -
VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_NONE - to mark that no type was
specified in the XML. This type is at the end converted to the standard
VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_SERIAL. Other types that are
different from this default have to be processed separately in the
device post parse callback.

tests/xml2vmxtest.c

@@ -15,11 +15,6 @@ static virCapsPtr caps;
 static virVMXContext ctx;
 static virDomainXMLOptionPtr xmlopt;
 
-static int testDefaultConsoleType(const char *ostype ATTRIBUTE_UNUSED,
-                                  virArch arch ATTRIBUTE_UNUSED)
-{
-    return VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_SERIAL;
-}
 
 static void
 testCapsInit(void)
@@ -32,8 +27,6 @@ testCapsInit(void)
         return;
     }
 
-    caps->defaultConsoleTargetType = testDefaultConsoleType;
-
     virCapabilitiesAddHostMigrateTransport(caps, "esx");