The virtual box driver was directly accesing the domain events
structs instead of using the APIs provided. To prevent this kind
of abuse, make the struct definitions private, forcing use of the
internal APIs. This requires adding one extra internal API.

* src/conf/domain_event.h, src/conf/domain_event.c: Move
  virDomainEventCallback and virDomainEvent structs into
  the source file instead of header
* src/vbox/vbox_tmpl.c: Use official APIs for dispatching domain
  events instead of accessing structs directly.

* global: patch created by running:
for f in $(git ls-files '*.[ch]') ; do
    cppi $f > $f.t && mv $f.t $f
done

Use the ATTRIBUTE_NONNULL annotation to mark some virConnectPtr
args as mandatory non-null so the compiler can warn of mistakes

* src/conf/domain_event.h: All virConnectPtr args must be non-null
* src/qemu/qemu_conf.h: qemudBuildCommandLine and
  qemudNetworkIfaceConnect() must be given non-null connection
* tests/qemuxml2argvtest.c: Provide a non-null (dummy) connection to
  qemudBuildCommandLine()

* src/capabilities.c, src/capabilities.h, src/domain_conf.c,
  src/domain_conf.h, src/domain_event.c, src/domain_event.h,
  src/interface_conf.c, src/interface_conf.h,
  src/network_conf.c, src/network_conf.h, src/node_device_conf.c,
  src/node_device_conf.h, src/secret_conf.c, src/secret_conf.h,
  src/storage_conf.c, src/storage_conf.h, src/storage_encryption_conf.c,
  src/storage_encryption_conf.h: Move to src/conf/
* src/Makefile.am: Add -Isrc/conf to the individual build targets
  which need to use XML config APIs. Remove LIBXML_CFLAGS, LIBSSH2_CFLAGS
  and SELINUX_CFLAGS from global INCLUDES and only have them in build
  targets which actually need them.  Create a libvirt_conf.la
  convenience library for all config parsers
* src/hostusb.h: Remove bogus include of domain_conf.h
* tests/Makefile.am: Add -Isrc/conf. Remove bogus -I$builddir/src
  since it never has any generated header files
* daemon/Makefile.am: Add -Isrc/conf
* proxy/Makefile.am: Add -Isrc/conf and cope with renamed files
* src/hash.c: Remove bogus include of libxml/threads.h