KVM: selftests: Clean up LIBKVM files in Makefile

Break up the long lines for LIBKVM and alphabetize each architecture.
This makes reading the Makefile easier, and will make reading diffs to
LIBKVM easier.

No functional change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220520233249.3776001-10-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>