vdpa: add vdpa device migration status ops

virt inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I86ITO



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Introduce a new interface to set vdpa device migration status, Such as
migration start/stop, pre_start/pre_stop, etc.

Some vdpa device need to do some job in different state. As not all vdpa
devices need to do this, this interface is optional.

Signed-off-by: jiangdongxu <jiangdongxu1@huawei.com>