virtio: acknowledge all features before access

virtio: acknowledge all features before access

The feature negotiation was designed in a way that
makes it possible for devices to know which config
fields will be accessed by drivers.

This is broken since commit 404123c2db79 ("virtio: allow drivers to
validate features") with fallout in at least block and net.
We have a partial work-around in commit 2f9a174f918e ("virtio: write
back F_VERSION_1 before validate") which at least lets devices
find out which format should config space have, but this
is a partial fix: guests should not access config space
without acknowledging features since otherwise we'll never
be able to change the config space format.

As a side effect, this also reduces the amount of hypervisor accesses -
we now only acknowledge features once unless we are clearing any
features when validating.

Cc: stable@vger.kernel.org
Fixes: 404123c2db79 ("virtio: allow drivers to validate features")
Fixes: 2f9a174f918e ("virtio: write back F_VERSION_1 before validate")
Cc: "Halil Pasic" <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>