migration: Report when bdrv_inactivate_all fails

migration: Report when bdrv_inactivate_all fails

If the bdrv_inactivate_all fails near the end of the migration,
the migration will fail and often the only diagnostics in the log
are an I/O error which you can't distinguish from an error on
the socket connection.

Add an error so we know when it's actually a block problem.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20170822170212[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>