tests: Fix broken ivshmem-server-msi/-irq tests

Broken with commit b4ba67d9a7025 ("libqos: Change PCI accessors to take
opaque BAR handle") a while ago, but nobody noticed since the tests are
not run by default: The msix_pba_bar is not correctly initialized
anymore if bir_pba has the same value as bir_table. With this fix,
"make check SPEED=slow" should work fine again.

Fixes: b4ba67d9a702507793c2724e56f98e9b0f7be02b
Tested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
