Add stream ID to MSI write

GICv3 ITS distinguishes between devices by using hardwired device IDs passed on the bus.
This patch implements passing these IDs in qemu.
SMMU is also known to use stream IDs, therefore this addition can also be useful for
implementing platforms with SMMU.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

 Changes from v1:
- Added bus number to the stream ID
- Added stream ID not only to MSI-X, but also to plain MSI. Some common code was made into
msi_send_message() function.
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/exec/memattrs.h

@@ -33,6 +33,8 @@ typedef struct MemTxAttrs {
     unsigned int secure:1;
     /* Memory access is usermode (unprivileged) */
     unsigned int user:1;
+    /* Stream ID (for MSI for example) */
+    unsigned int stream_id:16;
 } MemTxAttrs;
 
 /* Bus masters which don't specify any attributes will get this,