You can download H310 Mini driver for Windows 2008 R2 from below link. You can extract the content and provide using a USB key during installation.  [URL]   
I have the same issues with reinstalling windows server 2008r2 in dell PowerEdge r320 I have created a virtual disk but it can not find the drive to install the OS and my controller is PERC H310 Mini. [Please help I have been trying for 3 days now without any luck.
windows server 2008r2 on poweredge r320 failed to find drive