KVM: arm64: Safety check PSTATE when entering guest and handle IL

This commit adds a paranoid check when entering the guest to make sure
we don't attempt running guest code in an equally or more privilged mode
than the hypervisor.  We also catch other accidental programming of the
SPSR_EL2 which results in an illegal exception return and report this
safely back to the user.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>