ZFS install crash if more than one pool is created

[jibel]

In the manual partition page, if you create more than one ZFS partition, the installer crashes.

[xnox]

That's expected in curtin validation code:

    if len(zfsroots) > 1:
        raise ValueError(
            "zfsroot found in two entries in storage config: %s" % zfsroots)

I don't know how such validations are supposed to be verified in subiquity and new desktop installer.