migration: allow private destination ram with x-ignore-shared

By removing the share ram check, qemu is able to migrate
to private destination ram when x-ignore-shared capability
is on. Then we can create multiple destination VMs based
on the same source VM.

This changes the x-ignore-shared migration capability to
work similar to Lai's original bypass-shared-memory
work(https://lists.gnu.org/archive/html/qemu-devel/2018-04/msg00003.html)
which enables kata containers (https://katacontainers.io)
to implement the VM templating feature.

An example usage in kata containers(https://katacontainers.io):
1. Start the source VM:
   qemu-system-x86 -m 2G \
     -object memory-backend-file,id=mem0,size=2G,share=on,mem-path=/tmpfs/template-memory \
     -numa node,memdev=mem0
2. Stop the template VM, set migration x-ignore-shared capability,
   migrate "exec:cat>/tmpfs/state", quit it
3. Start target VM:
   qemu-system-x86 -m 2G \
     -object memory-backend-file,id=mem0,size=2G,share=off,mem-path=/tmpfs/template-memory \
     -numa node,memdev=mem0 \
     -incoming defer
4. connect to target VM qmp, set migration x-ignore-shared capability,
migrate_incoming "exec:cat /tmpfs/state"
5. create more target VMs repeating 3 and 4

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Yury Kotov <yury-kotov@yandex-team.ru>
Cc: Jiangshan Lai <laijs@hyper.sh>
Cc: Xu Wang <xu@hyper.sh>
Signed-off-by: Peng Tao <tao.peng@linux.alibaba.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1560494113-1141-1-git-send-email-tao.peng@linux.alibaba.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

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