msix: drop unused msix_bar_size, require valid bar_size

No user in sight for msix_bar_size.
bar_size for all users is aligned, let's simply
require this instead of trying to fix up invalid input.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>