qemu/msix: fix table access issues

Fixes a couple of issues with msix table access:
- With misbehaving guests, misaligned 4 byte access could overflow
  msix table and cause qemu to segfault. Since PCI spec requires
  host to only issue dword-aligned accesses, as a fix,
  it's enough to mask the address low bits.
- Tables use pci format, not native format, and so
  we must use pci_[sg]et_long on read/write.

Reported-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
