vhdx: Fix "log that needs to be replayed" error message

The arguments of error_setg_errno() should yield a short error string
without newlines.

Here, we try to append additional help to the error message by
embedding newlines in the error string.  That's nice, but it's doesn't
play nicely with the errno part.  tests/qemu-iotests/070.out shows the
resulting mess:

    can't open device TEST_DIR/iotest-dirtylog-10G-4M.vhdx: VHDX image file 'TEST_DIR/iotest-dirtylog-10G-4M.vhdx' opened read-only, but contains a log that needs to be replayed.  To replay the log, execute:
     qemu-img check -r all 'TEST_DIR/iotest-dirtylog-10G-4M.vhdx': Operation not permitted

Switch to error_setg() and error_append_hint().  Result:

    can't open device TEST_DIR/iotest-dirtylog-10G-4M.vhdx: VHDX image file 'TEST_DIR/iotest-dirtylog-10G-4M.vhdx' opened read-only, but contains a log that needs to be replayed
    To replay the log, run:
    qemu-img check -r all 'TEST_DIR/iotest-dirtylog-10G-4M.vhdx'

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-21-git-send-email-armbru@redhat.com>
