Used_memslots is equal to dev->mem->nregions now, it is true for
vhost kernel, but not for vhost user, which uses the memory regions
that have file descriptor. In fact, not all of the memory regions
have file descriptor.
It is usefully in some scenarios, e.g. used_memslots is 8, and only
5 memory slots can be used by vhost user, it is failed to hot plug
a new memory RAM because vhost_has_free_slot just returned false,
but we can hot plug it safely in fact.
Signed-off-by: Jinhua Cao <caojinhua1@huawei.com>
