qemu: bulk stats: Fix logic in monitor handling

A logic bug in qemuConnectGetAllDomainStats makes the code mark the
monitor as available when qemuDomainObjBeginJob fails, instead of when
it succeeds, as the correct flow requires.

This patch fixes the check and updates the code documentation
accordingly.

Broken by commit 57023c0a3a.

Signed-off-by: Francesco Romani <fromani@redhat.com>

src/qemu/qemu_driver.c

@@ -18745,9 +18745,9 @@ qemuConnectGetAllDomainStats(virConnectPtr conn,
         }
 
         if (HAVE_JOB(privflags) &&
-            qemuDomainObjBeginJob(driver, dom, QEMU_JOB_QUERY) < 0)
-            /* As it was never requested. Gather as much as possible anyway. */
+            qemuDomainObjBeginJob(driver, dom, QEMU_JOB_QUERY) == 0)
             domflags |= QEMU_DOMAIN_STATS_HAVE_JOB;
+        /* else: without a job it's still possible to gather some data */
 
         if (qemuDomainGetStats(conn, dom, stats, &tmp, domflags) < 0)
             goto endjob;