ZFS Install boot disk GPT check is broken for UEFI

[Jonathan Heathcote]

When performing a ZFS-based installation, curtin checks to see if the boot disk has a GPT-based partition table. On UEFI platforms, where the "grub_device" storage setting is set on the partition and not the disk, this check fails erroneously causing the installation to fail with a message such as:

    ValueError: zfsroot requires bootdisk with GPT partition table found "None" on disk id="uefi_boot_part"
    zfsroot requires bootdisk with GPT partition table found "None" on disk id="uefi_boot_part"

The offending code snippet appears to be commands/block_meta.py:1954

    # validate that the boot disk is GPT partitioned
    bootdevs = [d for i, d in storage_config.items() if d.get('grub_device')]
    bootdev = bootdevs[0]
    if bootdev.get('ptable') != 'gpt':
        raise ValueError(
            'zfsroot requires bootdisk with GPT partition table'
            ' found "%s" on disk id="%s"' %
            (bootdev.get('ptable'), bootdev.get('id')))

A correct implementation ought to check if the entry is a partition and, in that case, check the 'ptable' entry of the named disk (and not the partition entry of course).

For others encountering this issue, as a work-around you can add "grub_device: true" to your disk definition in addition to the relevant partition. On UEFI systems, the GRUB installation process silently ignores the disk-level flag making this workaround fairly harmless.