snp.sh: Update ubuntu guest image download to optimize the guest creation process

Updated the ubuntu guest download process to download it once and re-use the same image to quickly create multiple guests with the improved performance.
Update of the  guest image download structure in ubuntu also helps to  easily add support for the same in the other OS linux distros.

Signed-off-by: Harika Nittala <lnittala@amd.com>

Author: LakshmiSaiHarika

tools/snp.sh

@@ -98,6 +98,9 @@ SNPGUEST_URL="https://github.com/virtee/snpguest.git"
 SNPGUEST_BRANCH="tags/v0.8.0"
 NASM_SOURCE_TAR_URL="https://www.nasm.us/pub/nasm/releasebuilds/2.16.01/nasm-2.16.01.tar.gz"
 CLOUD_INIT_IMAGE_URL="https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-amd64.img"
+CLOUD_INIT_IMAGE_URL_UBUNTU="https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-amd64.img"
+IMAGE_BASENAME_UBUNTU=$(basename "${CLOUD_INIT_IMAGE_URL_UBUNTU}")
+IMAGE_BASENAME=""
 DRACUT_TARBALL_URL="https://github.com/dracutdevs/dracut/archive/refs/tags/059.tar.gz"
 SEV_SNP_MEASURE_VERSION="0.0.11"
 
@@ -503,6 +506,33 @@ generate_guest_ssh_keypair() {
   ssh-keygen -q -t ed25519 -N '' -f "${GUEST_SSH_KEY_PATH}" <<<y
 }
 
+download_guest_os_image(){
+  local linux_distro=$(get_linux_distro)
+
+  # Set the guest OS image-cloud init URL, guest image basename based on the Host OS type
+  case ${linux_distro} in
+    ubuntu)
+      CLOUD_INIT_IMAGE_URL=${CLOUD_INIT_IMAGE_URL_UBUNTU}
+      IMAGE_BASENAME=${IMAGE_BASENAME_UBUNTU}
+      ;;
+    *)
+      >&2 echo -e "ERROR: ${linux_distro}"
+      return 1
+      ;;
+  esac
+
+  local base_launch_directory=${LAUNCH_WORKING_DIR//"/$GUEST_NAME"*/}
+  local base_guest_image=${base_launch_directory}/${IMAGE_BASENAME}
+
+  # Download image if not present already
+  if [ ! -f ${base_guest_image} ]; then
+    wget "${CLOUD_INIT_IMAGE_URL}" -O ${base_guest_image}
+  fi
+
+  # Copy image to launch directory
+  cp -v ${base_guest_image} "${IMAGE}"
+}
+
 cloud_init_create_data() {
   if [[ -f "${LAUNCH_WORKING_DIR}/${GUEST_NAME}-metadata.yaml" && \
     -f "${LAUNCH_WORKING_DIR}/${GUEST_NAME}-user-data.yaml"  && \
@@ -541,8 +571,8 @@ EOF
     "${LAUNCH_WORKING_DIR}/${GUEST_NAME}-user-data.yaml" \
     "${LAUNCH_WORKING_DIR}/${GUEST_NAME}-metadata.yaml"
 
-  # Download ubuntu 20.04 and change name
-  wget "${CLOUD_INIT_IMAGE_URL}" -O "${IMAGE}"
+  # Download Guest Image from cloud init URL
+  download_guest_os_image
 }
 
 resize_guest() {