migration: Move migrate_use_zero_copy_send() to options.c

Once that we are there, we rename the function to
migrate_zero_copy_send() to be consistent with all other capabilities.

We can remove the CONFIG_LINUX guard.  We already check that we can't
setup this capability in migrate_caps_check().

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

migration/options.c

@@ -155,3 +155,12 @@ bool migrate_zero_blocks(void)
 
     return s->capabilities[MIGRATION_CAPABILITY_ZERO_BLOCKS];
 }
+
+bool migrate_zero_copy_send(void)
+{
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->capabilities[MIGRATION_CAPABILITY_ZERO_COPY_SEND];
+}