x86: add and use XEN_DOMCTL_getpageframeinfo3

x86: add and use XEN_DOMCTL_getpageframeinfo3

To support wider than 28-bit MFNs, add XEN_DOMCTL_getpageframeinfo3
(with the type replacing the passed in MFN rather than getting or-ed
into it) to properly back xc_get_pfn_type_batch().

With xc_get_pfn_type_batch() only used internally to libxc, move its
prototype from xenctrl.h to xc_private.h.

This also fixes a couple of bugs in pre-existing code:
- the failure path for init_mem_info() leaked minfo->pfn_type,
- one error path of the XEN_DOMCTL_getpageframeinfo2 handler used
  put_domain() where rcu_unlock_domain() was meant, and
- the XEN_DOMCTL_getpageframeinfo2 handler could call
  xsm_getpageframeinfo() with an invalid struct page_info pointer.

Signed-off-by: Jan Beulich <jbeulich@novell.com>