replay: provide an accessor for rr filename

replay: provide an accessor for rr filename

This patch adds an accessor function for the name of the record/replay
log file. Adding an accessor instead of making variable global,
prevents accidental modification of this variable by other modules.

Signed-off-by: Pavel Dovgalyuk <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <160174517710.12451.17645787545733927488.stgit@pasha-ThinkPad-X280>
Signed-off-by: Paolo Bonzini <[email protected]>