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>

Cargo.toml

@@ -43,6 +43,7 @@ acpi = ["vmm/acpi"]
 pci = ["vmm/pci_support"]
 mmio = ["vmm/mmio_support"]
 cmos = ["vmm/cmos"]
+fwdebug = ["vmm/fwdebug"]
 
 # Integration tests require a special environment to run in
 integration_tests = []