qga: introduce three guest memory block commmands with stubs

qga: introduce three guest memory block commmands with stubs

Introduce three new guest commands:
guest-get-memory-blocks, guest-set-memory-blocks, guest-get-memory-block-size.

With these three commands, we can support online/offline guest's memory block
(logical memory hotplug/unplug) as required from host.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
*generalized guest-get-memory-block-size to get-get-memory-block-info
 for future extensibility

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>