tp-qemu: Add cfg file for a SLOF(ppc only) case to boot with maxmem parameter.

qemu/tests/cfg/slof_with_maxmem.cfg

+- slof_with_maxmem: install setup image_copy unattended_install.cdrom
+    virt_test_type = qemu
+    type = boot
+    slots_mem = 4
+    restart_vm = yes
+    kill_vm_on_error = yes
+    login_timeout = 600
+    only ppc64 ppc64le
+    variants:
+        - 512G:
+            maxmem_mem = 512G
+        - 1024G:
+            maxmem_mem = 1024G