KVM: arm64: nv: Add switching support for HFGxTR/HDFGxTR

Now that we can evaluate the FGT registers, allow them to be merged
with the hypervisor's own configuration (in the case of HFG{RW}TR_EL2)
or simply set for HFGITR_EL2, HDGFRTR_EL2 and HDFGWTR_EL2.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Jing Zhang <jingzhangos@google.com>
Link: https://lore.kernel.org/r/20230815183903.2735724-26-maz@kernel.org