vmx: Adapt to emptyBackingString for cdrom-image

https://bugzilla.redhat.com/show_bug.cgi?id=1266088

We are missing this value for cdrom-image device. It seems like
there's no added value to extend this to other types of disk
devices [1].

1: https://www.redhat.com/archives/libvir-list/2016-January/msg01038.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

tests/xml2vmxdata/xml2vmx-cdrom-ide-empty.vmx

@@ -0,0 +1,13 @@
+.encoding = "UTF-8"
+config.version = "8"
+virtualHW.version = "4"
+guestOS = "other"
+uuid.bios = "56 4d 9b ef ac d9 b4 e0-c8 f0 ae a8 b9 10 35 15"
+displayName = "cdrom-ide-file"
+memsize = "4"
+numvcpus = "1"
+ide0:0.present = "true"
+ide0:0.deviceType = "cdrom-image"
+ide0:0.fileName = "emptyBackingString"
+floppy0.present = "false"
+floppy1.present = "false"