KVM: arm64: Add a vcpu flag to control ptrauth for guest

A per vcpu flag is added to check if pointer authentication is
enabled for the vcpu or not. This flag may be enabled according to
the necessary user policies and host capabilities.

This patch also adds a helper to check the flag.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@arm.com>
Cc: kvmarm@lists.cs.columbia.edu
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>