Add memory paging support for MMU updates (mapping a domain's memory).

Add memory paging support for MMU updates (mapping a domain's memory).

If Domain-0 tries to map a page that has been paged out, then propagate an
error so that it knows to try again. If the page is paged out, request that
it be paged back in. If the page is in the process of being paged in, then
just keeping returning the error until it is paged back in.

This requires the co-operation of the Domain-0 kernel's privcmd mmap
functions. The kernel can't simply spin waiting for the page, as this will
cause a dead-lock (since the paging tool lives in Domain-0 user-space and if
it's spinning in kernel space, it will never return to user-space to allow the
page to be paged back in). There is a complimentary Linux patch which sees
ENOENT, which is not returned by any other part of this code, and marks the
PFN of that paged specially to indicate it was paged out (much like what it
does with PFNs that are within the range of a domain's memory but are not
presently mapped).

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