qemu: fix build when python3 is enabled

qemu: fix build when python3 is enabled

Fixes:
http://autobuild.buildroot.net/results/04e/04ec455e352319a46dc06def1762c65ad28a12a1/
http://autobuild.buildroot.net/results/f19/f1932657912674af444602ac59211b6caf6841ca/
http://autobuild.buildroot.net/results/18a/18a43df80616280506a44ac3abc327ddf0e7b079/
http://autobuild.buildroot.net/results/44c/44c00b50248018c4e81f4fbb1041d35b78f89640/

And many others.

When python3 for the target is enabled, $(HOST_DIR)/usr/bin/python points to
host-python3, not host-python - Breaking the build. Ensure host-python is used.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>