libvirt functions returning "unit" correspond to C fns which use -1 for failure

This affects the following functions:
  virStoragePoolBuild
  virStoragePoolDelete
  virStorageVolDelete

Previously a call to virStorageVolDelete would succeed returning 0, which
was interpreted as false, causing us to raise an exception with
VIR_ERR_NONE.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>