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, yeah, why need we to encapsulate extra layer/class/function to customize our error?  we can get a clear error from command output.