KVM: arm64: Implement vq_present() as a macro

This routine is a one-liner and doesn't really need to be function and
can be implemented as a macro.

Suggested-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>