Ignore empty type attribute in driver element of virtual disks

Fixes http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=578347

src/qemu/qemu_conf.c

@@ -2479,7 +2479,7 @@ qemuBuildDriveStr(virDomainDiskDefPtr disk,
     if (disk->readonly &&
         qemuCmdFlags & QEMUD_CMD_FLAG_DEVICE)
         virBufferAddLit(&opt, ",readonly=on");
-    if (disk->driverType &&
+    if (disk->driverType && *disk->driverType != '\0' &&
         disk->type != VIR_DOMAIN_DISK_TYPE_DIR &&
         qemuCmdFlags & QEMUD_CMD_FLAG_DRIVE_FORMAT)
         virBufferVSprintf(&opt, ",format=%s", disk->driverType);