msix: Prevent bogus mask updates on MMIO accesses

>From: Jan Kiszka <jan.kiszka@siemens.com>

Only accesses to the MSI-X table must trigger a call to
msix_handle_mask_update, otherwise the vector
value might be out of range.

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