qemu: Const-correct snapshot directory name

qemuDomainSnapshotWriteMetadata does not modify the directory name,
and making it const-correct aids in writing an upcoming patch.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_domain.c

@@ -8399,7 +8399,7 @@ qemuDomainSnapshotWriteMetadata(virDomainObjPtr vm,
                                 virDomainSnapshotObjPtr snapshot,
                                 virCapsPtr caps,
                                 virDomainXMLOptionPtr xmlopt,
-                                char *snapshotDir)
+                                const char *snapshotDir)
 {
     char *newxml = NULL;
     int ret = -1;

src/qemu/qemu_domain.h

@@ -683,7 +683,7 @@ int qemuDomainSnapshotWriteMetadata(virDomainObjPtr vm,
                                     virDomainSnapshotObjPtr snapshot,
                                     virCapsPtr caps,
                                     virDomainXMLOptionPtr xmlopt,
-                                    char *snapshotDir);
+                                    const char *snapshotDir);
 
 int qemuDomainSnapshotForEachQcow2(virQEMUDriverPtr driver,
                                    virDomainObjPtr vm,