fix: make mem_size_mib and track_dirty_pages optional

This makes these fields optional in PATCH /machine-config requests. The
comment on this structure says that all fields should be optional, and I
dont quite see why these two should be different. Thus, add
`serde(default)` to avoid forcing customers to explicitly set them to
`null` if they do not want to update these parts of the machine config.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

CHANGELOG.md

@@ -28,6 +28,9 @@ and this project adheres to
 - [#4916](https://github.com/firecracker-microvm/firecracker/pull/4916): Fixed
   `IovDeque` implementation to work with any host page size. This fixes
   virtio-net device on non 4K host kernels.
+- [#????](<>): Fixed `mem_size_mib` and `track_dirty_pages` being mandatory for
+  all `PATCH /machine-config` requests. Now, they can be omitted which leaves
+  these parts of the machine configuration unchanged.
 
 ## [1.10.1]
 

src/vmm/src/vmm_config/machine_config.rs

@@ -136,6 +136,7 @@ pub struct MachineConfigUpdate {
     #[serde(default)]
     pub vcpu_count: Option<u8>,
     /// The memory size in MiB.
+    #[serde(default)]
     pub mem_size_mib: Option<usize>,
     /// Enables or disabled SMT.
     #[serde(default)]
@@ -144,6 +145,7 @@ pub struct MachineConfigUpdate {
     #[serde(default)]
     pub cpu_template: Option<StaticCpuTemplate>,
     /// Enables or disables dirty page tracking. Enabling allows incremental snapshots.
+    #[serde(default)]
     pub track_dirty_pages: Option<bool>,
     /// Configures what page size Firecracker should use to back guest memory.
     #[serde(default)]