vfio/migration: obtain the mailbox configuration at one time

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7BJW9


CVE: NA

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The mailbox configuration(128 bits) needs to be obtained from
the hardware at one time. If the mailbox configuration is
obtained for multiple times, the read value may be incorrect.
Use the instruction to read mailbox data instead of readl().

Fixes: a0464f0b ("vfio/hisilicon: add acc live migration driver")
Signed-off-by: Weili Qian <qianweili@huawei.com>
Signed-off-by: JiangShui Yang <yangjiangshui@h-partners.com>