xenpaging: unify return value in nominate and evict

xenpaging: unify return value in nominate and evict

Let p2m_mem_paging_nominate and p2m_mem_paging_evict return just one
error number. EINVAL is not very helpful in case of nominate, it can
happen if the pager tries to nominate a ballooned page. In this case the
gfn is not backed by a mfn, the pager can not know that.  Similar with
evict, anything can happen between nominate and evict.

This change helps the pager to decide if the returned error is from the
function itself, or if it happend earlier. In the latter case, it is
most likely fatal and should be handled as such.
nominate and evict return EBUSY, which is supposed to mean
"pager request reached target function, and failed."

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>