Allow setting bootfs property on pools with indirect vdevs

The FreeBSD boot loader relies on the bootfs property and is capable
of booting from removed (indirect) vdevs.

Reviewed-by Eric van Gyzen
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #11763