backport exposing FLUSHBYASID when running nested VMs on AMD CPUs

backport exposing FLUSHBYASID when running nested VMs on AMD CPUs

this exposes the FLUSHBYASID CPU flag to nested VMs when running on an
AMD CPU. also reverts a made up check that would advertise
FLUSHBYASID as not supported. this enable certain modern hypervisors
such as VMWare ESXi 7 and Workstation 17 to run nested VMs properly
again.

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>
(cherry picked from commit 3202de9857143bb619f91aa5fae28f950db09647)
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>