virCaps: get rid of macPrefix field

Use the virDomainXMLConf structure to hold this data and tweak the code
to avoid semantic change.

Without configuration the KVM mac prefix is used by default. I chose it
as it's in the privately administered segment so it should be usable for
any purposes.

tests/xml2vmxtest.c

@@ -34,7 +34,6 @@ testCapsInit(void)
 
     caps->defaultConsoleTargetType = testDefaultConsoleType;
 
-    virCapabilitiesSetMacPrefix(caps, (unsigned char[]){ 0x00, 0x0c, 0x29 });
     virCapabilitiesAddHostMigrateTransport(caps, "esx");