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>
Can we define another exception for SELinux?
Such as SELinuxError or SELinuxBooleanSetupError.
As an API of SELinux, people may not want to get TestError,
because they may use this function in env_process or somewhere.