virttest.env_process: move close serial_console session step

move close serial_console session in the end of postprocess as it'll help
keeping the number of filedescriptors used by virt test.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1515 from xutian/small_fix

virttest.env_process: move close serial_console session step