loader.efi: chain loader should provide proper device handle

[tsoome]

Summary

Since the efipart rewrite, the chain command was looking for device
handle using interface applicable only for net devices. Disk
partitions and zfs pools need their own approach to find the proper handle.

Test Plan

tested by chain loading loader.efi from another zfs pool and from ESP.

[imp]

This looks good to me.

[sylvain_sylvaingarrigues.com]

This breaks the kernel build if WITHOUT_ZFS is defined in src.conf.