libxc support of memory paging.

libxc support of memory paging.

libxc accepts the new return code from privcmd mmap, which indicates a page
being mapped is actually paged out. Spin until the page is paged in and return
as normal to the caller. This allows memory paging to work transparently with
existing tools.

Since libxc runs in user-space, as does the pager, both processes will be
scheduled and run. This enables the page to be paged in without needing to
spin in kernel mode (which would cause a dead-lock).

Signed-off-by: Patrick Colp <Patrick.Colp@citrix.com>