tests/vm: fix and simplify HOST_ARCH definition

tests/vm: fix and simplify HOST_ARCH definition

ARCH is always empty, so just define HOST_ARCH as the result of uname.
The incorrect definition was not being used because the "ifeq" statement
is wrong; replace it with the same idiom based on $(realpath) that the
main Makefile uses.

With this change, vm-build-netbsd in a configured tree will not use
the PYTHONPATH hack.

Reported-by: John Snow <jsnow@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>