feat(vmm): address bus, interrupt sender, and x86_64 legacy devices (M1 slice) #1585

Implements M1 device-model tasks for x86_64 per docs/architecture/vmm/README.md: Bus/BusDevice with overlap rejection, IrqSender over set_irq_line (edge = set+clear), 8250 at 0x3F8 (GSI 4), CMOS RTC at 0x70/0x71, i8042 at 0x60/0x64 with 0xFE -> reset flag. Pure userspace, fake-backed unit tests; does not touch the KVM backend (#1576) or vCPU threads. Reference: libkrun legacy devices (e12b9b3) / Firecracker 68698ad devices/legacy.