Use Cstruct.t for buffers rather than Io_page.t, for convenience

Note there is a workaround for the Xen gntshr stubs-- the stub will
dereference the Cstruct and grant the 'base_page_of' the whole Cstruct.
This means that if you have a 2-page Cstruct, and try to grant both
pages, both grants will grant the same page only.

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