template: mark efidisk as read-only

template: mark efidisk as read-only

otherwise backups of templates using UEFI fail with storages like LVM
thin, where the volumes are not writable. disk controllers like IDE and
SATA that don't support being read-only are still broken for UEFI.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
[ drop the readonly=off when not required, resolve merger conflict
  from Dominik's EFI disk cache mode fix ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>