qemu: move setting emulatorpin ahead of monitor showing up

qemu: move setting emulatorpin ahead of monitor showing up

If VM is configured with many devices(including passthrough devices)
and large memory, libvirtd will take seconds(in the worst case) to
wait for monitor. In this period the qemu process may run on any
PCPU though I intend to pin emulator to the specified PCPU in xml
configuration.

Actually qemu process takes high cpu usage during vm startup.
So this is not the strict CPU isolation in this case.

Signed-off-by: Zhou yimin <zhouyimin@huawei.com>