arm64: KVM: Treat sysreg accessors returning false as successful

Instead of considering that a sysreg accessor has failed when
returning false, let's consider that it is *always* successful
(after all, we won't stand for an incomplete emulation).

The return value now simply indicates whether we should skip
the instruction (because it has now been emulated), or if we
should leave the PC alone if the emulation has injected an
exception.

Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>