KVM: selftests: Gracefully handle empty stack traces

Bail out of test_dump_stack() if the stack trace is empty rather than
invoking addr2line with zero addresses. The problem with the latter is
that addr2line will block waiting for addresses to be passed in via
stdin, e.g. if running a selftest from an interactive terminal.

Opportunistically fix up the comment that mentions skipping 3 frames
since only 2 are skipped in the code.

Cc: Vipin Sharma <vipinsh@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220922231724.3560211-1-dmatlack@google.com>
[Small tweak to keep backtrace() call close to if(). - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>