KVM: arm64: pkvm: Preserve pending SError on exit from AArch32

Don't drop a potential SError when a guest gets caught red-handed
running AArch32 code.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20211013120346.2926621-8-maz@kernel.org