Since this is a generic function that implements bitarray logic move
it from kvm to utils so that it can be easily used across all modules.

Make it raise errors.GenericError if it cannot find a free slot in
the given bitarray.

We add this function in a separate utils module (bitarrays) that can be
extended in the future. Currently only the network module and the
KVM hypervisor make use of bitarrays. This patch is a step forward
unifying these pieces of code and make both use the same bitarray
utility functions.

Add unittest for bitarrays utils module.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Hrvoje Ribicic <riba@google.com>