Don't attempt trimming "hole" vdevs

On zpools containing hole vdevs (e.g. removed log devices), the `zpool
trim` (and presumably zpool initialize) commands will attempt calling
their respective functions on "hole", which fails, as this is not a real
vdev.

Avoid this by removing HOLE vdevs in zpool_collect_leaves.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Niklas Haas <git@haasn.xyz>
Closes #10227