L4Linux: fix usage of l4_touch_xx (fix #682)

Use size in bytes, when touching memory via l4_touch_ro or l4_touch_rw, instead
of log2 size.
L4Linux stability improvements

- search for alternative virtual address regions upwards, starting from
  the given start address, in the 'l4re_rm_attach()' and
  'Region_manager::reserve_range()' functions

- don't treat memory locations above 0x80000000 in l4linux's virtual
  address space as device memory

- align the start address of the vmalloc area according to the assumption
  in 'devicemaps_init()'

Fixes #414.
Merge branch 'staging' of github.com:Ksys-labs/genode into staging