KVM: arm/arm64: Provide functionality to pause and resume a guest

For some rare corner cases in our VGIC emulation later we have to stop
the guest to make sure the VGIC state is consistent.
Provide the necessary framework to pause and resume a guest.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>