vmm: default GDB to false when deserializing

This fixes the valid VM config unit tests, which would otherwise fail
to deserialize their expected JSON config due to the missing "gdb" field.

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

vmm/src/vm_config.rs

@@ -595,6 +595,7 @@ pub struct VmConfig {
     #[serde(default)]
     pub watchdog: bool,
     #[cfg(feature = "guest_debug")]
+    #[serde(default)]
     pub gdb: bool,
     pub platform: Option<PlatformConfig>,
     pub tpm: Option<TpmConfig>,