vmm, devices: Add firmware debug port device

OVMF and other standard firmwares use I/O port 0x402 as a simple debug
port by writing ASCII characters to it. This is gated under a feature
that is not enabled by default.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/Cargo.toml

@@ -10,6 +10,7 @@ acpi = ["acpi_tables","devices/acpi"]
 pci_support = ["pci", "vfio", "vm-virtio/pci_support"]
 mmio_support = ["vm-virtio/mmio_support"]
 cmos = ["devices/cmos"]
+fwdebug = ["devices/fwdebug"]
 
 [dependencies]
 arc-swap = ">=0.4.4"