[Libguestfs] resize: Preserve GPT GUID so we don't break EFI bootloaders (RHBZ#1189284)

virt-resize didn't preserve the per-partition GPT GUID. Now that guests using UEFI are becoming common (basically it's the default on aarch64) we need to take into account that sometimes the partition GUID is used by the bootloader NVRAM variables to identify the boot partition, so it must be preserved across resize. This bug caused the 'virt-builder --size' option to fail on aarch64. Rich.