Do not try to use MBR on AArch64

When I tried to build CentOS8 image for AArch64 I got error saying that
MBR is not supported. So make sure that it will not be used by default.

Change-Id: Ib67ab7f808d727c3c61932c540d398dbe723972f

diskimage_builder/lib/common-functions

@@ -302,7 +302,11 @@ function _arg_defaults_hack() {
     done
 
     if [[ -n "${vm_seen}" && -z "${blockdev_seen}" ]]; then
-        elements="$elements block-device-mbr"
+        if [[ "arm64 aarch64" =~ $ARCH ]] ; then
+            elements="$elements block-device-efi"
+        else
+            elements="$elements block-device-mbr"
+        fi
     fi
 
     echo $elements