virtio: update MemoryRegionCaches when guest negotiates features

virtio: update MemoryRegionCaches when guest negotiates features

Because the cache is sized to include the rings and the event indices,
negotiating the VIRTIO_RING_F_EVENT_IDX feature will result in the size
of the cache changing.  And because MemoryRegionCache accesses are
range-checked, if we skip this we end up with an assertion failure.
This happens with OpenBSD 6.3.

Reported-by: Fam Zheng <[email protected]>
Fixes: 97cd965c070152bc626c7507df9fb356bbe1cd81
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>
Tested-by: Fam Zheng <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>