tests: Make api_create_body() parameterless

Refactor api_create_body() to read cpu_count, kernel_path,
and kernel_cmdline from Guest fields instead of taking them
as parameters. This makes Guest the single source of truth
for VM configuration.

Update all call sites in HTTP and DBus API tests to use the
new parameterless signature. Switch guest creation to use
GuestFactory for consistent 4-CPU configuration.

Replace manual CPU and memory assertions with
validate_cpu_count() and validate_memory() helpers.
Replace thread::sleep with wait_vm_boot() in
_test_api_create_boot for proper boot synchronization.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

test_infra/src/lib.rs

@@ -1083,11 +1083,11 @@ impl Guest {
         )
     }
 
-    pub fn api_create_body(&self, cpu_count: u8, kernel_path: &str, kernel_cmd: &str) -> String {
+    pub fn api_create_body(&self) -> String {
         let mut body = serde_json::json!({
             "cpus": {
-            "boot_vcpus": cpu_count,
-            "max_vcpus": cpu_count,
+            "boot_vcpus": self.num_cpu,
+            "max_vcpus": self.num_cpu,
             },
             "net": [
             {
@@ -1117,13 +1117,13 @@ impl Guest {
                 .unwrap()
                 .to_str()
                 .unwrap(),
-            "cmdline": kernel_cmd,
+            "cmdline": self.kernel_cmdline.as_deref().unwrap(),
             "host_data": generate_host_data(),
             });
         } else {
             body["payload"] = serde_json::json!({
-            "kernel": kernel_path,
-            "cmdline": kernel_cmd,
+            "kernel": self.kernel_path.as_deref().unwrap(),
+            "cmdline": self.kernel_cmdline.as_deref().unwrap(),
             });
         }