utils_libguestfs: Unset color environment for guestfish test.

For some Linux release(RHEL7.1), characters will have color
when we enter guestfish shell session. It may cause failure
of testcases. So covering it during testing.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Maybe you can put these env_params into a list, so it more easy to read and maintain