msi: Guard msi/msix_write_config with msi_present

Terminate msi/msix_write_config early if support is not enabled. This
allows to remove checks at the caller site if MSI is optional.

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