vmm: cpu: Fix slow vector initialization

warning: slow zero-filling initialization
    --> vmm/src/cpu.rs:1780:9
     |
1779 |         let mut mat_data: Vec<u8> = Vec::new();
     |                                     ---------- help: consider replacing this with: `vec![0; std::mem::size_of_val(&lapic)]`
1780 |         mat_data.resize(std::mem::size_of_val(&lapic), 0);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
     = note: `#[warn(clippy::slow_vector_initialization)]` on by default

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

vmm/src/cpu.rs

@@ -1776,8 +1776,7 @@ impl Cpu {
             _reserved: 0,
         };
 
-        let mut mat_data: Vec<u8> = Vec::new();
-        mat_data.resize(std::mem::size_of_val(&lapic), 0);
+        let mut mat_data: Vec<u8> = vec![0; std::mem::size_of_val(&lapic)];
         // SAFETY: mat_data is large enough to hold lapic
         unsafe { *(mat_data.as_mut_ptr() as *mut LocalX2Apic) = lapic };