MFV 331704:
9191 dump vdev tree to zfs_dbgmsg when spa load fails due to missing log devices

illumos/illumos-gate@ccef24b493bcbd146fcd6d8946666cae081470b6

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>