qemu-char: check errno together with ret < 0

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 <[email protected]>
Message-Id: <20180704033642[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Fixes: 9fc53a10f81d3a9027b23fa810147d21be29e614
Signed-off-by: Paolo Bonzini <[email protected]>