chardev: fix handling of EAGAIN for TCP chardev

When this commit was applied

  commit 9894dc0c
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Tue Jan 19 11:14:29 2016 +0000

    char: convert from GIOChannel to QIOChannel

The tcp_chr_recv() function was changed to return QIO_CHANNEL_ERR_BLOCK
which corresonds to -2. As such the handling for EAGAIN was able to be
removed from tcp_chr_read(). Unfortunately in a later commit:

  commit b6572b4f
  Author: Marc-André Lureau <marcandre.lureau@redhat.com>
  Date:   Fri Mar 11 18:55:24 2016 +0100

    char: translate from QIOChannel error to errno

The tcp_chr_recv() function was changed back to return -1, with errno
set to EAGAIN, without also re-addding support for this to tcp_chr_read()

Reported-by: Aleksey Kuleshov <rndfax@yandex.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180222121351.26191-1-berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>