virt: Remove '7za' command check in bootstrap module

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
I think '7za' is not a essential command for it's only for JeOS's download. If a user just wants to run a testcase, '7za' is not necessary for him. So I modify that the '7za' will be checked in JeOS's download branch.