xen-blkback/sysfs: Move the parameters for the persistent grant features

to a testing subdirectory (as this value should not be baked
for the life-time) and also in an appropiate file.

Also modified the introduction Linux version from 3.10 to 3.11.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>