msix: fix msix_vector_masked

commit 428c3ece97179557f2753071fb0ca97a03437267 ("fix MSI injection on Xen")
inadvertently enabled the xen-specific logic unconditionally.
Limit it to only when xen is enabled.
Additionally, msix data should be read with pci_get_log
since the format is pci little-endian.

Reported-by: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
