gptzfsboot: "ZFS: i/o error - all block copies unavailable" after crash

(original report: https://lists.freebsd.org/pipermail/freebsd-virtualization/2016-April/004362.html)

I am running r298620 as dom0 under Xen, for debugging
some Xen-related crashes.

Two drives, ada0 and ada1 in a mirror configuration
(swap via gmirror):

=>        34  5860533101  ada0  GPT  (2.7T)
          34        1024     1  freebsd-boot  (512K)
        1058     4194304     2  freebsd-swap  (2.0G)
     4195362  5856337773     3  freebsd-zfs  (2.7T)

  pool: zroot
 state: ONLINE
  scan: scrub repaired 0 in 0h7m with 0 errors on Mon Apr 25 19:22:30 2016
config:

    NAME        STATE     READ WRITE CKSUM
    zroot       ONLINE       0     0     0
      mirror-0  ONLINE       0     0     0
        ada0p3  ONLINE       0     0     0
        ada1p3  ONLINE       0     0     0


After a crash, the bootblocks complain on boot

ZFS: i/o error - all block copies unavailable
ZFS: i/o error - all block copies unavailable
ZFS: i/o error - all block copies unavailable

Can't find /boot/zfsloader

FreeBSD/x86 boot
Default: zroot:/boot/kernel/kernel
boot:
ZFS: i/o error - all block copies unavailable

(...)

When I boot a rescue system (10.3) via PXE
I can import the pool without any problems
(using "zpool import -R /mnt zroot").

Installing 10.3 /boot/gptzfsboot
in the ada0p1/ada1p1 boot blocks makes the
problem go away - I can crash the system as many
times I like and the next boot works fine.

Just restoring -current /boot/gptzfsboot
does not usually help.

Marcin