xenconfig: xm: check for driver on disk format

When reviewing libxl vif typename series[0] I found a bug
on xen-xm formatter where "virsh domxml-to-native xen-xm file.xml"
can lead to a NULL dereference if the disk driver isn't specified.
Fix this by checking for driver before writing/testing it down.

[0] https://www.redhat.com/archives/libvir-list/2016-April/msg01434.html

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>

src/xenconfig/xen_xm.c

@@ -297,9 +297,12 @@ xenFormatXMDisk(virConfValuePtr list,
                 type = "aio";
             else
                 type = virStorageFileFormatTypeToString(format);
+
+            if (driver) {
                 virBufferAsprintf(&buf, "%s:", driver);
                 if (STREQ(driver, "tap"))
                     virBufferAsprintf(&buf, "%s:", type);
+            }
         } else {
             switch (virDomainDiskGetType(disk)) {
             case VIR_STORAGE_TYPE_FILE: