At the moment, only build these softmmu emulators for x86_64 and sunxi
target.  The decision was made for the following reasons

 - It seems that interests of virtualization with qemu are mostly from
   x86, and ARM recently.
 - x86, sunxi boards/boxes capable of running qemu with accel=kvm are more
   widely available
 - Not all host, target combinations of qemu works, or even compiles
 - Extra maintenance work and server resources

Test results are as the following

 - Nested vmx works: lede-qemu-x86_64-kvm on lede-qemu-x86_64-kvm
 - KVM on Cubieboard2 works
 - tcg with malta works: lede-qemu-malta-tcg on lede-qemu-malta-tcg.
   But it's too slow to be useful thus not included in this version
 - mips64 host does not compile

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>