block: mirror - use local_err to avoid NULL errp

When starting a block job, commit_active_start() relies on whether *errp
is set by mirror_start_job.  This allows it to determine if the mirror
job start failed, so that it can clean up any changes to open flags from
the bdrv_reopen().  If errp is NULL, then it will not be able to
determine if mirror_start_job failed or not.

To avoid this, use a local Error variable, and then propagate the error
(if any) to errp.

Reported-by: Markus Armbruster <armbru@redhat.com>
block: Don't throw away errno via error_setg

There are a handful of places in the block layer where a failure path
has a valid -errno value, yet error_setg() is used.  Those instances
should instead use error_setg_errno(), to preserve as much error
information as possible.

This patch replaces those instances with error_setg_errno(), so that
errno is passed up the stack in the error message.

Reported-By: Kevin Wolf <kwolf@redhat.com>
block: qemu-iotests - fix test 070 (vhdx)

VHDX test 070 failed, due to different output from qemu-io / qemu
when opening an image read-only that contains a log file.  Filter
the output, and update the expected results to match the correct
output.
block: qemu-iotests - add vhdx log replay tests for qemu-img

VHDX logs can now be replayed via 'qemu-img check -r all'.  Add
tests to verify that the log replay is successful when using qemu-img.