cpu: Add support for CPU vendor

By specifying <vendor> element in CPU requirements a guest can be
restricted to run only on CPUs by a given vendor. Host CPU vendor is
also specified in capabilities XML.

The vendor is checked when migrating a guest but it's not forced, i.e.,
guests configured without <vendor> element can be freely migrated.

tests/testutilsqemu.c

@@ -83,6 +83,7 @@ virCapsPtr testQemuCapsInit(void) {
         0,                      /* match */
         (char *) "x86_64",      /* arch */
         (char *) "core2duo",    /* model */
+        (char *) "Intel",       /* vendor */
         1,                      /* sockets */
         2,                      /* cores */
         1,                      /* threads */