tests: Don't assume structure of PCI IO base in ahci-test

In a couple of places ahci-test makes assumptions about how the tokens
returned from qpci_iomap() are formatted in ways it probably shouldn't.

First in verify_state() it uses a non-NULL token to indicate that the AHCI
device has been enabled (part of enabling is to iomap()).  This changes it
to use an explicit 'enabled' flag instead.

Second, it uses the fact that the token contains a PCI address, stored when
the BAR is mapped during initialization to check that the BAR has the same
value after a migration.  This changes it to explicitly read the BAR
register before and after the migration and compare.

Together, these changes will  make the test more robust against changes to
the internals of the libqos PCI layer.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
