Error handling: Audit callers of load_image_targphys, get_image_size, event_notifier_init, msix_init

These functions (load_image_targphys, get_image_size, event_notifier_init, msix_init) return a negative value on error. Audit callers of these functions and ensure that they:

(1) Can cope with these function calls failing gracefully, (2) Check for ret < 0 instead of ret != 0.