docs: add comment for VCpuSet type in vmm.rs

Author: aarkegz

src/vmm.rs

@@ -74,6 +74,7 @@ pub type InterruptVector = u8;
 /// The maximum number of virtual CPUs supported in a virtual machine.
 pub const MAX_VCPU_NUM: usize = 64;
 
+/// A set of virtual CPUs.
 pub type VCpuSet = cpumask::CpuMask<MAX_VCPU_NUM>;
 
 /// The API trait for virtual machine management functionalities.