sheepdog: Consistently set bdrv_has_zero_init_truncate

sheepdog: Consistently set bdrv_has_zero_init_truncate

block_int.h claims that .bdrv_has_zero_init must return 0 if
.bdrv_has_zero_init_truncate does likewise; but this is violated if
only the former callback is provided if .bdrv_co_truncate also exists.
When adding the latter callback, it was mistakenly added to only one
of the three possible sheepdog instantiations.

Fixes: 1dcaf527
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200324174233.1622067-5-eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>