Can I grant a user ALTER TRACE permission to a single database?
i doubt that 

the ALTER TRACE permission is a server level permission, and access is at the server level; if a user can start a trace, he or she can retrieve event data no matter what database the event was generated in.

for more info

http://technet.microsoft.com/en-us/library/cc293611.aspx

http://msdn.microsoft.com/en-us/library/ms187611.aspx