qemu: driver: Fix error message in qemuDomainBlockCommit

When qemu does not support changing of the backing store string, we'd
reaport that block pull is not supported instead of block commit.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_driver.c

@@ -17705,7 +17705,7 @@ qemuDomainBlockCommit(virDomainPtr dom,
         topSource != disk->src) {
         if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_CHANGE_BACKING_FILE)) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                           _("this qemu doesn't support relative blockpull"));
+                           _("this qemu doesn't support relative block commit"));
             goto endjob;
         }