Add support for PMEM Namespace for instance with accelerator

Currently, in Nova we need configure "pmem_namespaces=$LABEL:$NSNAME[|$NSNAME][,$LABEL:$NSNAME[|$NSNAME]]" in nova.conf, and configure flavor extra specs "hw:pmem=$LABEL[,$LABEL]", then we can use this flavor booting an instance[1].
For this, we only provide the PMEM Namespaces to instance at booting, for a booted instance it can not use PMEM Namespace in cuurent method. It's not friendly for users.

[1] https://specs.openstack.org/openstack/nova-specs/specs/train/implemented/virtual-persistent-memory.html