refactor(pcie): avoid string format on good path

If a function is called in the `.ok_or`, this gets executed
independently on whether the function succeeds or not, although the
result is only used on failure.

Replace it with a .ok_or_else to avoid a useless string allocation and
memcpy.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

src/vmm/src/vstate/vm.rs

@@ -170,7 +170,7 @@ impl InterruptSourceGroup for MsiVectorGroup {
 
     fn trigger(&self, index: InterruptIndex) -> vm_device::interrupt::Result<()> {
         self.notifier(index)
-            .ok_or(std::io::Error::other(format!(
+            .ok_or_else(|| std::io::Error::other(format!(
                 "trigger: invalid interrupt index {index}"
             )))?
             .write(1)