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>

tests/vm/Makefile.include

@@ -1,14 +1,12 @@
 # Makefile for VM tests
 
 # Hack to allow running in an unconfigured build tree
-ifeq ($(wildcard $(SRC_PATH)/config-host.mak),)
+ifeq ($(realpath $(SRC_PATH)),$(realpath .))
 VM_PYTHON = PYTHONPATH=$(SRC_PATH)/python /usr/bin/env python3
 VM_VENV =
-HOST_ARCH := $(shell uname -m)
 else
 VM_PYTHON = $(TESTS_PYTHON)
 VM_VENV = check-venv
-HOST_ARCH = $(ARCH)
 endif
 
 .PHONY: vm-build-all vm-clean-all
@@ -23,6 +21,7 @@ ARM64_IMAGES += ubuntu.aarch64 centos.aarch64
 endif
 endif
 
+HOST_ARCH = $(shell uname -m)
 ifeq ($(HOST_ARCH),x86_64)
 IMAGES=$(X86_IMAGES) $(if $(USE_TCG),$(ARM64_IMAGES))
 else ifeq ($(HOST_ARCH),aarch64)