qemu: Clarify error message in qemuMigrationSrcIsSafe

The original message was logically incorrect: cache != none or cache !=
directsync is always true. But even replacing "or" with "and" doesn't
make it more readable for humans.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Acked-By: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_migration.c

@@ -1266,7 +1266,7 @@ qemuMigrationSrcIsSafe(virDomainDefPtr def,
 
         virReportError(VIR_ERR_MIGRATE_UNSAFE, "%s",
                        _("Migration may lead to data corruption if disks"
-                         " use cache != none or cache != directsync"));
+                         " use cache other than none or directsync"));
         return false;
     }