Grant kernel access to new "virtual_disk" file.

This is a special file that can be mounted as a loopback device to
exercise adoptable storage code on devices that don't have valid
physical media.  For example, they may only support storage media
through a USB OTG port that is being used for an adb connection.

avc: denied { read } for path="/data/misc/vold/virtual_disk" dev="sda35" ino=508695 scontext=u:r:kernel:s0 tcontext=u:object_r:vold_data_file:s0 tclass=file permissive=0

Bug: 34903607
Change-Id: I84721ec0e9495189a7d850461875df1839826212