KVM: arm64: Restructure FGT register switching

As we're about to majorly extend the handling of FGT registers,
restructure the code to actually save/restore the registers
as required. This is made easy thanks to the previous addition
of the EL2 registers, allowing us to use the host context for
this purpose.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230815183903.2735724-14-maz@kernel.org