accel/hvf: Implement hvf_arch_vcpu_destroy()

accel/hvf: Implement hvf_arch_vcpu_destroy()

Call hv_vcpu_destroy() to destroy our vCPU context.

As hv_vcpu_destroy() must be called by the owning thread,
document hvf_arch_vcpu_destroy() also does.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Mads Ynddal <mads@ynddal.dk>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>