Open VCPU interrupts while the VCPU is in an IPC operation can badly influence
its state. That's why this commit fixes a whole bunch of places, where IRQs
weren't masked by now.
In Genode's paravirtualized block driver a request cache is used to find again
Linux request structures via corresponding block-session packets. To work
correctly the cache needs to have at least the same size like the
block-session's queue-size.
Due to recently introduces smart-pointers to Cap_index objects it's
necessary to always keep at least one reference as long as a corresponding
slot in the capability-space of a process is in use. This is especially
important for L4Linux that uses cap-slots directly without the given
abstractions of Genode.
