This field was needed to differentiate memory slots created by the new
API, KVM_SET_USER_MEMORY_REGION, from those by the old equivalent,
KVM_SET_MEMORY_REGION, whose support was dropped long before:

  commit b74a07be
  KVM: Remove kernel-allocated memory regions

Although we also have private memory slots to which KVM allocates
memory with vm_mmap(), !user_alloc slots in other words, the slot id
should be enough for differentiating them.

Note: corresponding function parameters will be removed later.
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>