hw/i386/vmmouse: Require 'i8042' property to be set

hw/i386/vmmouse: Require 'i8042' property to be set

If the 'i8042' property is not set, mouse events handled by
vmmouse_mouse_event() end calling i8042_isa_mouse_fake_event()
with a NULL argument, resulting in ps2_mouse_fake_event() being
called with invalid PS2MouseState pointer. Fix by requiring
the 'i8042' property to be always set:

  $ qemu-system-x86_64 -device vmmouse
  qemu-system-x86_64: -device vmmouse: 'i8042' link is not set

Fixes: 91c9e09147b ("vmmouse: convert to qdev")
Reported-by: Calvin Buckley <calvin@cmpct.info>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/752
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211201223253.36080-1-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>