xen-block: Use specific blockdev driver

... when a xen-block backend instance is created via xenstore.

Following 8d17adf3 ("block: remove support for using "file" driver
with block/char devices"), using the "file" blockdev driver for
everything doesn't work anymore, we need to use the "host_device"
driver when the disk image is a block device and "file" driver when it
is a regular file.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Paul Durrant <paul@xen.org>
Message-Id: <20210430163432.468894-1-anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

hw/block/xen-block.c

@@ -728,6 +728,8 @@ static XenBlockDrive *xen_block_drive_create(const char *id,
     XenBlockDrive *drive = NULL;
     QDict *file_layer;
     QDict *driver_layer;
+    struct stat st;
+    int rc;
 
     if (params) {
         char **v = g_strsplit(params, ":", 2);
@@ -761,7 +763,17 @@ static XenBlockDrive *xen_block_drive_create(const char *id,
     file_layer = qdict_new();
     driver_layer = qdict_new();
 
-    qdict_put_str(file_layer, "driver", "file");
+    rc = stat(filename, &st);
+    if (rc) {
+        error_setg_errno(errp, errno, "Could not stat file '%s'", filename);
+        goto done;
+    }
+    if (S_ISBLK(st.st_mode)) {
+        qdict_put_str(file_layer, "driver", "host_device");
+    } else {
+        qdict_put_str(file_layer, "driver", "file");
+    }
+
     qdict_put_str(file_layer, "filename", filename);
     g_free(filename);