Add new API virDomainBlockPull* to headers

Add new API virDomainBlockPull* to headers

Set up the types for the block pull functions and insert them into the
virDriver structure definition.  Symbols are exported in this patch to prevent
documentation compile failures.

* include/libvirt/libvirt.h.in: new API
* src/driver.h: add the new entry to the driver structure
* python/generator.py: fix compiler errors, the actual python bindings are
  implemented later
* src/libvirt_public.syms: export symbols

Signed-off-by: Adam Litke <agl@us.ibm.com>