vmm: remove Default impls for config

These Default implementations either don't produce valid configs, are
no longer used outside of tests, or both.

For the tests, we can define our own local "default" values that make
the most sense for the tests, without worrying about what's
a (somewhat) sensible "global" default value.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

src/main.rs

@@ -853,7 +853,11 @@ mod unit_tests {
             },
             payload: Some(PayloadConfig {
                 kernel: Some(PathBuf::from("/path/to/kernel")),
-                ..Default::default()
+                firmware: None,
+                cmdline: None,
+                initramfs: None,
+                #[cfg(feature = "igvm")]
+                igvm: None,
             }),
             rate_limit_groups: None,
             disks: None,