[litevirt] move keep out of litevirt minimizer script

Litevirt minimizer script cannot be used for keeping anything, so
move them out of the script.
[litevirt] Keep openvswitch kernel modules

They are required to provide vswitch support for litevirt.
[litevirt] Keep i2c driver modules

They are required by igb since kernel 3.9, so keep them to support
igb ethernet adapters.
[litevirt] Fix a chcon complain during boot up

When selinux is disabled, chcon complains at the system boot up.

Check whether selinux is enabled before make a call to chcon.