qemu: Fix compilation error when enum variable size differs from 'int'

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.