qtest: fix qtest log fd should be initialized before qtest chardev

qtest_log_fp should be inited before qemu_chr_add_handlers.
If not the log dumped from callback functions may be lost.

easy to reproduce it by command:
"QTEST_LOG=1 QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64
gtester -k --verbose -m=quick tests/qdev-monitor-test"

The log "[I xxxxxx] OPENED" should be printed out by
qtest_event, but does not.

Signed-off-by: Li Liu <john.liuli@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
