os.linux: Move the HostThread class from lttng2.kernel to os.linux

os.linux: Move the HostThread class from lttng2.kernel to os.linux

It is a class that can be used for other analyzes than only the virtual machine
one.

Change-Id: Icd417d68d41c6b6363fe440ab7cd38d9ab4ad9d2
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/39897
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>