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: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>