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, I know your meaning, but I indeed don't want to define a customized error to override original error, in here, it's not just a TestError, it also includes the original error in error log.