selftests/kvm: fix compilation on non-x86_64 platforms

MEM_REGION_SLOT and MEM_REGION_GPA are not really needed in
test_invalid_memory_region_flags; the VM never runs and there are no
other slots, so it is okay to use slot 0 and place it at address
zero.  This fixes compilation on architectures that do not
define them.

Fixes: 5d743164

 ("KVM: selftests: Add a memory region subtest to validate invalid flags")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>