Caller of CharDriverState.chr* callback assume errno error conventions.
Translate QIOChannel error to errno (this fixes potential EAGAIN
regression, for ex if a vhost-user backend block, qemu_chr_fe_read_all()
could get error -2 and not wait)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1457718924-19338-1-git-send-email-marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>