vdpa: add device state operations

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



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Introduce several interfaces to allow vdpa device save/load device
status when guest machine resume and suspend.

The get_dev_buffer_size interface is used to get the buffer size
of vdpa device status.

The get_dev_buffer interface is used to get the device buffer from
vdpa device, and VMM can save it.

The set_dev_buffer interface is used to set the device status from
userspace.

These operations are optional. If they are not implemented, return
EOPNOTSUPP.

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