test_infra: set default kernel path and cmdline in Guest

Initialize kernel_path and kernel_cmdline with standard
direct boot defaults in the Guest constructor instead of
None. This removes boilerplate from individual tests that
use the common direct kernel boot configuration.

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

test_infra/src/lib.rs

@@ -985,8 +985,8 @@ impl Guest {
             network,
             vm_type: GuestVmType::Regular,
             boot_timeout: DEFAULT_TCP_LISTENER_TIMEOUT,
-            kernel_path: None,
-            kernel_cmdline: None,
+            kernel_path: direct_kernel_boot_path().to_str().map(String::from),
+            kernel_cmdline: Some(DIRECT_KERNEL_BOOT_CMDLINE.to_string()),
             console_type: None,
             num_cpu: 1u32,
             nested: true,