builder: templates: For RHEL, CentOS >= 7 and all Fedora, default to GPT.

The immediate issue is with Fedora/ppc64 and /ppc64le which currently
use extended partitions, breaking the virt-builder ‘--size’ parameter,
eg:

  $ virt-builder --arch ppc64le fedora-26 --size 20G
  ...
  [  21.6] Resizing (using virt-resize) to expand the disk to 20.0G
  virt-resize: error: /dev/sda5: partition not found in the source disk image
  (this error came from '--expand' option on the command line).  Try running
  this command: virt-filesystems --partitions --long -a /var/tmp/vbf67b8c.img

However more generally MBR is broken and should die.  GPT is supported
by all modern virtual bootloaders, so just default to it.

Notes:

* This is different from mandating a UEFI bootloader.

* I am not planning to rebuild any existing images except the
  F26 ppc64 & ppc64le ones.

builder/templates/make-template.ml

@@ -481,7 +481,7 @@ part /     --fstype=%s   --size=1024 --grow --asprimary
    | CentOS _ | RHEL _ | Fedora _ ->
       bpf "\
 zerombr
-clearpart --all --initlabel
+clearpart --all --initlabel --disklabel=gpt
 autopart --type=plain
 ";
    | _ -> assert false (* cannot happen, see caller *)