x86,kvm: Use the new workload event to trigger KVM system calls.

x86,kvm: Use the new workload event to trigger KVM system calls.

While events are only used for SE mode for now, this moves to using the
common mechanism and gets rid of the need for the system call specific
pseudo inst.

Change-Id: I53468103d7f046b85cc25cbff94b12dbc946f4f0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34163
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>