Created attachment 358427 [details]
check whether src is not null, then insert the file directive

Description of problem:
qemu-kvm no longer accepts '-drive' option in the following form 'file=,if=...' with 'file' sub-option pointing to nowhere.
consequently a vm fails to start. 

Version-Release number of selected component (if applicable):
qemu-kvm head: bee37f32d47a6c04ff57d18e01cf296607e22b30
libvirt head: 777fc2e9d60844a7387355d9cef06bd25190d146

How reproducible:

always

Steps to Reproduce:
1. define a vm with cdrom drive
2. disconnect it from any source
3. try to start the vm
  
Actual results:

fails to start

Expected results:
runs smoothly

Additional info:

a quick fix attached