vhost-vdpa: clean iotlb map during reset for older userspace

Using .compat_reset op from the previous patch, the buggy .reset
behaviour can be kept as-is on older userspace apps, which don't ack the
IOTLB_PERSIST backend feature. As this compatibility quirk is limited to
those drivers that used to be buggy in the past, it won't affect change
the behaviour or affect ABI on the setups with API compliant driver.

The separation of .compat_reset from the regular .reset allows
vhost-vdpa able to know which driver had broken behaviour before, so it
can apply the corresponding compatibility quirk to the individual driver
whenever needed.  Compared to overloading the existing .reset with
flags, .compat_reset won't cause any extra burden to the implementation
of every compliant driver.

[mst: squashed in two fixup commits]

Message-Id: <[email protected]>
Message-Id: <[email protected]>
Reported-by: Dragos Tatulea <[email protected]>
Tested-by: Dragos Tatulea <[email protected]>
Message-Id: <[email protected]>
Reported-by: Lei Yang <[email protected]>
Signed-off-by: Si-Wei Liu <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Tested-by: Lei Yang <[email protected]>

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