vmm: Add acpi table for fw_cfg device

This allows the fw_cfg device to be recognized by the guest linux
kernel. This becomes more relavnt in the following cl where I add
the option to load files into the guest via fw_cfg. The Linux kernel
already has a fw_cfg driver that will automatically load these files
under /sys when CONFIG_FW_CFG_SYSFS is enabled in the kernel config

For arm we must add fw_cfg to the devices tree

Signed-off-by: Alex Orozco <alexorozco@google.com>

arch/src/lib.rs

@@ -155,6 +155,9 @@ pub enum DeviceType {
     /// Device Type: GPIO.
     #[cfg(target_arch = "aarch64")]
     Gpio,
+    /// Device Type: fw_cfg.
+    #[cfg(feature = "fw_cfg")]
+    FwCfg,
 }
 
 /// Default (smallest) memory page size for the supported architectures.