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..., because I think utils_misc and some other modules like utils_net should be framework APIs. It should be different from utils_test and test modules.
And I want to keep it pure as base APIs. So it means TestError or likewise should be only appear in utils_test and test modules.
What do you think?