s390: Detect invalid invocations of qemu_ram_free/remap

This both detects invalid invocations of qemu_ram_free and
qemu_ram_remap when mem_path is non-NULL and fixes a build error on
s390 ("'area' may be used uninitialized in this function").

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
CC: Alexander Graf <agraf@suse.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
