The call to drv->bdrv_reopen_prepare() can fail due to reasons
other than an open failure. Unfortunately, we can't use errno
nor -ret, cause they are not always set.

Stick to a generic error message then.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Acked-by: NKevin Wolf <kwolf@redhat.com>