Convert an error from guest_cpu_read() or guest_cpu_write() into a vdev_ref_status value
#include <qvm/vdev-core.h>
static enum vdev_ref_status guest_cpu_to_vrs(int const errcode)
It is assumed that the returned value is an error.
VRS_BUS_ERROR if errcode is -1; otherwise, VRS_ERRNO and the errcode value.