lib: use an argument which doesn't require default argument promotion

Clang warns that passing an object that undergoes default argument
promotion to "va_start" is undefined behavior:

lib/report.c:106:15: error: passing an object that undergoes default
argument promotion to 'va_start' has undefined behavior
[-Werror,-Wvarargs]
        va_start(va, pass);

Using an "unsigned" type removes the need for argument promotion.

Signed-off-by: Bill Wendling <morbo@google.com>
Message-Id: <CAGG=3QUL_OrjaWn+gF4z-R8brR2=3661hGk0uUAK2y8Dff7Mvg@mail.gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>