coverity-model: Fix replay_get_byte()

Coverity 2018.06 chokes on replay_get_byte():

    $ cov-make-library -of scripts/coverity-model.xmldb scripts/coverity-model.c
    output file: scripts/coverity-model.xmldb
    Compiling scripts/coverity-model.c with command /opt/cov-sa-2018.06/bin/cov-emit --dir /tmp/cov-armbru/930a6fb31e5f464fc1a53354b2deb66b/cov-make-library-emit -w --no_error_recovery --emit_header_functions --no_implicit_decl --preinclude /opt/cov-sa-2018.06/library/decls.h --c scripts/coverity-model.c
    "scripts/coverity-model.c", line 110: error #20: identifier "replay_file" is
              undefined
           if (replay_file) {
               ^

    Emit for file '/work/armbru/qemu/scripts/coverity-model.c' complete.
    [ERROR] 1 error detected in the compilation of "scripts/coverity-model.c".
    ERROR: cov-emit returned with code 1

Broken in commit 04a0afe5.  Fix by dumbing down.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180626085642.4973-1-armbru@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>