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>
Hmm, do you mean 'result' passed to TestError?
Can we format it into self-defined SELinuxError?
Like this:
```
class SELinuxError(Exception):
    def __init__(self, bool_var, bool_value, reason):
   ...
```