terminal3270: do not use backend timer sources

terminal3270: do not use backend timer sources

terminal3270 uses the front-end side of the chardev. It shouldn't
create sources from backend side context (with backend
functions).

send_timing_mark_cb calls qemu_chr_fe_write_all() which should be
thread safe.

This partially reverts changes from commit
2c716ba1506769c9be2caa02f0f6d6e7c00f4304.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190206174328.9736-4-marcandre.lureau@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>