Introduce new API for generating random numbers

Introduce new API for generating random numbers

The old virRandom() API was not generating good random numbers.
Replace it with a new API virRandomBits which instead of being
told the upper limit, gets told the number of bits of randomness
required.

* src/util/virrandom.c, src/util/virrandom.h: Add virRandomBits,
  and move virRandomInitialize
* src/util/util.h, src/util/util.c: Delete virRandom and
  virRandomInitialize
* src/libvirt.c, src/security/security_selinux.c,
  src/test/test_driver.c, src/util/iohelper.c: Update for
  changes from virRandom to virRandomBits
* src/storage/storage_backend_iscsi.c: Remove bogus call
  to virRandomInitialize & convert to virRandomBits