typing string via VNC is unreliable

QEMU version is 3.0.0

# Description

The problem is that, when typing string through VNC, it can be unreliable -- sometimes some key strokes get skipped, sometimes get swapped, sometimes get repeated.  There's no problem when typing through VNC on physical hardware.

# Steps to reproduce

1. Launch virtual machine by:

    qemu-kvm -display vnc=:1 -m 2048 opensuse-leap-15.qcow2

2. Connect to VNC by:

    vncviewer -Shared :5901

3. Simulate a series of key strokes by "vncdotool" [1]:

    vncdotool -s 127.0.0.1::5901 typefile strings_to_be_typed.txt

4. Usually after a few hundred keys are typed, something goes wrong.

I attached a screenshot that it mistypes " hello" to "h ello".

[1] https://github.com/sibson/vncdotool