KVM: arm64: vgic-v3: Allow userspace to write GICD_TYPER2.nASSGIcap

KVM: arm64: vgic-v3: Allow userspace to write GICD_TYPER2.nASSGIcap

KVM unconditionally advertises GICD_TYPER2.nASSGIcap (which internally
implies vSGIs) on GICv4.1 systems. Allow userspace to change whether a
VM supports the feature. Only allow changes prior to VGIC initialization
as at that point vPEs need to be allocated for the VM.

For convenience, bundle support for vLPIs and vSGIs behind this feature,
allowing userspace to control vPE allocation for VMs in environments
that may be constrained on vPE IDs.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250724062805.2658919-5-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>