spapr_vty: New case to test spapr-vty device to login and hot-unplug

New case to test spapr-vty device to login and hot-unplug.
Signed-off-by: NZhenyu Zhang <zhenyzha@redhat.com>

qemu/tests/cfg/spapr_vty_console_login.cfg

+- spapr_vty_console_login:
+    only ppc64 ppc64le
+    type = chardev_pci_serial_login
+    serial_type = spapr-vty
+    start_vm = no