virttest.utils_misc: add SELinux boolean manager

We usually need to set SELinux Boolean value on the local and remote host, it will be convenient for testers if we have a class for managing SELinux Boolean value, for example, we should enable 'virt_use_nfs' on the local and remote host firstly if we do a VM migration with a NFS sharing storage.

Signed-off-by: Alex Jia <ajia@redhat.com>
@yumingfei, sometimes, it may be a thankless thing, if you do this, it means you need to know all of existing errors then define your error(reason), it hasn't any help for actually underlying debugging, moreover, the extra calling layer also can't provide a good performance.