getdomaininfolist: return Val_int, not Int_val

This caused the "int" which was supposed to be 0 to not be
equal to the literal 0.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
userspace: Io_page.get takes pages, not bytes

Listing domains in batches of 512 should be adequate (not batches of
2M)

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