qemu_guest_agent: Adopt guest 'python' command to python3

Signed-off-by: Nqizhu <qizhu@redhat.com>

qemu/tests/cfg/qemu_guest_agent.cfg

@@ -77,7 +77,7 @@
             gagent_check_type = set_vcpus
         - check_get_time:
             gagent_check_type = get_time
-            get_guest_time_cmd = python -c "import time; print(int(time.time()))"
+            get_guest_time_cmd = `command -v python python3 | head -1` -c "import time; print(int(time.time()))"
         - check_memory_leak:
             only Windows
             gagent_check_type = memory_leak
@@ -87,7 +87,7 @@
         - check_set_time:
             image_snapshot = yes
             gagent_check_type = set_time
-            get_guest_time_cmd = python -c "import time; print(int(time.time()))"
+            get_guest_time_cmd = `command -v python python3 | head -1` -c "import time; print(int(time.time()))"
             move_time_cmd = "date --rfc-3339=seconds --utc; date --set='now - 1 week' > /dev/null; date --rfc-3339=seconds --utc"
         - check_fstrim:
             no Windows