i440fx: avoid destroying memory regions within a transaction

Calling memory_region_destroy() within a transaction is illegal, since
the memory API is allowed to continue to dispatch to a region until the
transaction commits.  440fx does that however when managing PAM registers.

This bug is benign, since the regions are all aliases (which the memory
core tends to throw anyway), and since we don't do concurrent dispatch yet,
but instead of relying on that, tighten ship ahead of the coming concurrency
storm.

Fix by having a predefined set of regions, of which one will be enabled at
any time.

Signed-off-by: Avi Kivity <avi@redhat.com>
