devices: Add fw_cfg cli options

This allows us to enable/disable the fw_cfg device via the cli

We can also now upload files into the guest vm using fw_cfg_items
via the cli

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

vmm/src/lib.rs

@@ -2391,6 +2391,8 @@ mod unit_tests {
                 igvm: None,
                 #[cfg(feature = "sev_snp")]
                 host_data: None,
+                #[cfg(feature = "fw_cfg")]
+                fw_cfg_config: None,
             }),
             rate_limit_groups: None,
             disks: None,