KVM: arm64: fix misleading comments in save/restore

The elr_el2 and spsr_el2 registers in fact contain the processor state
before entry into EL2. In the case of guest state it could be in either
el0 or el1.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>