Clean the env before vm clone

There could be scenario that existing VMs with the same name which block
the following operations. Remove these VMs to clean the env.

Signed-off-by: Yalan Zhang <yalzhang@redhat.com>

libvirt/tests/src/daemon/conf_file/sysconfig_libvirt_guests/libvirt_guests.py

@@ -308,8 +308,14 @@ def transfer_to_transient(per_guest_name):
         utils_path.find_command("virt-clone")
     except utils_path.CmdNotFoundError:
         test.cancel("No virt-clone command found.")
-
     # Clone additional vms: avocado-vt-vm2, avocado-vt-vm3.....
+    # Before clone, make sure there is no existing VMs
+    all_vm_list = virsh.dom_list('--name --all').stdout.strip().splitlines()
+    logging.debug("All vm on the host: %s", all_vm_list)
+    for vm_i in all_vm_list:
+        if vm_i.startswith("main_vm_name") and vm_i != "main_vm_name":
+            logging.debug("Clean the env by remove the guest : %s", vm_i)
+            virsh.remove_domain(vm_i, "--remove-all-storage --nvram")
     for i in range(additional_vms):
         guest_name = ("%s" % main_vm_name[:-1])+("%s" % str(i+2))
         logging.debug("guest_name : %s", guest_name)