pc-dimm: error out if memory hotplug is not enabled

fixes QEMU abort in case it's started without memory
hotplug enabled.

as result of fix it will print following messages:
"
-device pc-dimm,id=d1,memdev=m1: memory hotplug is not enabled, enable it on startup
-device pc-dimm,id=d1,memdev=m1: Device 'pc-dimm' could not be initialized
"

Also fixup assert condition to detect hotplug address
space overflow.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reported-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>