All our useful entry points into the hypervisor are starting by
saving x0 and x1 on the stack. Let's move those into the vectors
by introducing macros that annotate whether a vector is valid or
not, thus indicating whether we want to stash registers or not.

The only drawback is that we now also stash registers for el2_error,
but this should never happen, and we pop them back right at the
start of the handling sequence.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>