qemu: Make qemuDomainCheckDiskStartupPolicy self-contained

There is an error reset following the function and check for
startupPolicy before that.  Let's reflect those things inside that
function so that future code doesn't have to be that complex.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_domain.c

@@ -4192,6 +4192,7 @@ qemuDomainCheckDiskStartupPolicy(virQEMUDriverPtr driver,
                 return -1;
             break;
 
+        case VIR_DOMAIN_STARTUP_POLICY_DEFAULT:
         case VIR_DOMAIN_STARTUP_POLICY_MANDATORY:
             return -1;
 
@@ -4200,14 +4201,13 @@ qemuDomainCheckDiskStartupPolicy(virQEMUDriverPtr driver,
                 return -1;
             break;
 
-        case VIR_DOMAIN_STARTUP_POLICY_DEFAULT:
         case VIR_DOMAIN_STARTUP_POLICY_LAST:
             /* this should never happen */
             break;
     }
 
     qemuDomainCheckRemoveOptionalDisk(driver, vm, diskIndex);
-
+    virResetLastError();
     return 0;
 }
 
@@ -4241,12 +4241,8 @@ qemuDomainCheckDiskPresence(virQEMUDriverPtr driver,
         if (qemuDomainDetermineDiskChain(driver, vm, disk, true, true) >= 0)
             continue;
 
-        if (disk->startupPolicy &&
-            qemuDomainCheckDiskStartupPolicy(driver, vm, idx,
-                                             cold_boot) >= 0) {
-            virResetLastError();
+        if (qemuDomainCheckDiskStartupPolicy(driver, vm, idx, cold_boot) >= 0)
             continue;
-        }
 
         goto error;
     }