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 but return 1, which
would cause us to raise an exception with VIR_ERR_NONE.

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