qemu-char: check errno together with ret < 0

In the tcp_chr_write function, we checked errno,
but errno was not reset before a read or write operation.
Therefore, this check of errno's actions is often
incorrect after EAGAIN has occurred.
we need check errno together with ret < 0.

Signed-off-by: xinhua.Cao <caoxinhua@huawei.com>
Message-Id: <20180704033642.15996-1-caoxinhua@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Fixes: 9fc53a10
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>