KVM: selftests: Link selftests directly with lib object files

The linker does obey strong/weak symbols when linking static libraries,
it simply resolves an undefined symbol to the first-encountered symbol.
This means that defining __weak arch-generic functions and then defining
arch-specific strong functions to override them in libkvm will not
always work.

More specifically, if we have:

lib/generic.c:

  void __weak foo(void)
  {
          pr_info("weak\n");
  }

  void bar(void)
  {
          foo();
  }

lib/x86_64/arch.c:

  void foo(void)
  {
          pr_info("strong\n");
  }

And a selftest that calls bar(), it will print "weak". Now if you make
generic.o explicitly depend on arch.o (e.g. add function to arch.c that
is called directly from generic.c) it will print "strong". In other
words, it seems that the linker is free to throw out arch.o when linking
because generic.o does not explicitly depend on it, which causes the
linker to lose the strong symbol.

One solution is to link libkvm.a with --whole-archive so that the linker
doesn't throw away object files it thinks are unnecessary. However that
is a bit difficult to plumb since we are using the common selftests
makefile rules. An easier solution is to drop libkvm.a just link
selftests with all the .o files that were originally in libkvm.a.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220520233249.3776001-9-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>