packer: Let PATH find qemu executable

There is no need to use absolute path to executable, as PATH environment
variable provide one and it is easy to be overriden by the user.

packer/util/quick-test

@@ -4,4 +4,4 @@
 echo "Changes will not be written"
 echo "To force write please use:"
 echo "man qemu: You can however force the write back by pressing C-a s."
-/usr/bin/qemu-system-x86_64 $2 -snapshot -drive file=$1,if=virtio,cache=writeback,discard=ignore -boot once=d -m 512M -name test -machine type=pc,accel=kvm -netdev user,id=user.0,hostfwd=tcp::3213-:22 -device virtio-net,netdev=user.0 -vnc 0.0.0.0:47
+qemu-system-x86_64 $2 -snapshot -drive file=$1,if=virtio,cache=writeback,discard=ignore -boot once=d -m 512M -name test -machine type=pc,accel=kvm -netdev user,id=user.0,hostfwd=tcp::3213-:22 -device virtio-net,netdev=user.0 -vnc 0.0.0.0:47