scons: Make the USE_KVM variable available in C++.
scons: Make the USE_KVM variable available in C++.

We need it to determine whether we should expect KVM related parameters
exist in the cirrus graphics device.
(transplanted from b3ea7444f46666f020332d1c6fe8635aa81f719a)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5