vancouver: run script for booting Linux VMs

The setup now uses nitpicker and nit_fb to display several instances of
vancouver. The guest OS binaries must be supplied in the
'<build-dir>/bin' directory manually.

Furthermore, the patch lets launchpad pass Block, Nic, and Rtc to the
parent.
vancouver: keep one cap_session

Cap_sessions and portals created via the sessions are nowadays freed up during
c++ object destruction. Because of that the exception portals for a vCPU thread
get be revoked as soon as the cap_session object leaves its scope.

Keep one cap_session for the whole lifetime of the vmm to avoid disappearing
exception portals.

Related to #582.
vancouver: coding style
l4linux.run: increase timeout for test