[libvirt] adding tests to qemu_driver

[noxdafox]

Greetings,

I was investigating on an issue in which QEMU's dynamic ownership was 
not properly working when calling qemuDomainCoreDumpWithFormat().

The core of the issue seems to be the qemuOpenFileAs() function which 
does not handle the dynamic ownership. This might affect other libvirt's 
features within as well.

The function is quite complicated and I was thinking about refactoring 
it a bit in order to simplify the logic allowing to fix the dynamic 
ownership handling as well.

In order to refactor it, I was planning to write tests in order to cover 
all its use cases before actually changing the code.

The issue is that all the functions within the qemu_driver.c module are 
static. I could indeed include the module itself in my tests but I'm not 
sure whether this is acceptable.

Furthermore I'd like to have some clarification about the NFS related 
code. It seems that some effort has been put in order to tackle 
something I'm not aware of. Could someone briefly explain how to 
reproduce NFS failing scenarios?

Thank you.