qemu: Fix compilation error when enum variable size differs from 'int'
    
    Since commit bcd9a564b631aa virDomainNumatuneGetMode returns the value
    via a pointer rather than in the return value. The change triggered
    problems with platforms where the compiler decides to use a data type of
    size different than integer at the point where we typecast it.
    
    Work around the issue by using an intermediate variable of the correct
    type that gets casted back by the default typecasting rules.