qemu: Adjust the prototype to match the function

Changing the prototype to not have "int *index" since we'll soon be
disallowing index as a name. Curiously the original commit (a4504ac1)
for the function used 'int idx' in the function - so they didn't match.
Now they do.

src/qemu/qemu_conf.h

@@ -288,7 +288,7 @@ typedef qemuSharedDeviceEntry *qemuSharedDeviceEntryPtr;
 
 bool qemuSharedDeviceEntryDomainExists(qemuSharedDeviceEntryPtr entry,
                                        const char *name,
-                                       int *index)
+                                       int *idx)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 char *qemuGetSharedDeviceKey(const char *disk_path)