bhyve: cdrom support

Add support for CDROM devices for bhyve driver using
bhyve(8)'s 'ahci-cd' device type.

As bhyve currently does not support media insertion at runtime,
disallow to start a domain with an empty source path for cdrom
devices.

tests/bhyvexml2argvtest.c

@@ -104,6 +104,7 @@ mymain(void)
 
     DO_TEST("base");
     DO_TEST("acpiapic");
+    DO_TEST("disk-cdrom");
     DO_TEST("disk-virtio");
     DO_TEST("macaddr");
     DO_TEST("serial");