accel/hvf: Rename hvf_vcpu_exec() -> hvf_arch_vcpu_exec()

accel/hvf: Rename hvf_vcpu_exec() -> hvf_arch_vcpu_exec()

hvf_vcpu_exec() is implemented per target, rename it as
hvf_arch_vcpu_exec(), following the per target pattern.

Since it calls hv_vcpu_run(), mention it must be called
on the vCPU.

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>