Add managed save API entry points

Add managed save API entry points

virDomainManagedSave() is to be run on a running domain. Once the call
complete, as in virDomainSave() the domain is stopped upon completion,
but there is no restore counterpart as any order to start the domain
from the API would load the state from the managed file, similary if
the domain is autostarted when libvirtd starts.
Once a domain has restarted his managed save image is destroyed,
basically managed save image can only exist for a stopped domain,
for a running domain that would be by definition outdated data.

* include/libvirt/libvirt.h.in src/libvirt.c src/libvirt_public.syms:
  adds the new entry points virDomainManagedSave(),
  virDomainHasManagedSaveImage() and virDomainManagedSaveRemove()
* src/driver.h src/esx/esx_driver.c src/lxc/lxc_driver.c
  src/opennebula/one_driver.c  src/openvz/openvz_driver.c
  src/phyp/phyp_driver.c src/qemu/qemu_driver.c src/vbox/vbox_tmpl.c
  src/remote/remote_driver.c src/test/test_driver.c src/uml/uml_driver.c
  src/xen/xen_driver.c: add corresponding new internal drivers entry
  points