misc: Change cpu ID type from u8 to u32

This is the first change to Cloud Hypervisor in a series of changes
intended to increase the max number of supported vCPUs in guest VMs,
which is currently limited to 255 (254 on x86_64).

No user-visible/behavior changes are expected as a result of
applying this patch, as the type of boot_cpus and related
fields in config structs remains u8 for now, and all configuration
validations remain the same.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Neel Natu <neelnatu@google.com>
Signed-off-by: Ofir Weisse <oweisse@google.com>
Signed-off-by: Peter Oskolkov <posk@google.com>

fuzz/fuzz_targets/http_api.rs

@@ -221,7 +221,7 @@ impl RequestHandler for StubApiRequestHandler {
         Ok(())
     }
 
-    fn vm_resize(&mut self, _: Option<u8>, _: Option<u64>, _: Option<u64>) -> Result<(), VmError> {
+    fn vm_resize(&mut self, _: Option<u32>, _: Option<u64>, _: Option<u64>) -> Result<(), VmError> {
         Ok(())
     }