KVM: arm64: Turn host HVC handling into a dispatch table

Now that we can use function pointer, use a dispatch table to call
the individual HVC handlers, leading to more maintainable code.

Further improvements include helpers to declare the mapping of
local variables to values passed in the host context.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>