accel/hvf: Guard hv_vcpu_run() between cpu_exec_start/end() calls

accel/hvf: Guard hv_vcpu_run() between cpu_exec_start/end() calls

Similarly to 1d78a3c3ab8 for KVM, wrap hv_vcpu_run() with
cpu_exec_start/end(), so that the accelerator can perform
pending operations while all vCPUs are quiescent. See also
explanation in commit c265e976f46 ("cpus-common: lock-free
fast path for cpu_exec_start/end").

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