libxl: Remove pointless hypercall from libxl_set_memory_target

libxl: Remove pointless hypercall from libxl_set_memory_target

There's no obvious reason for the call to xc_domain_getinfolist -- all
it seems to be doing is checking that the domain exists; but if it
doesn't exist, it will have already failed by this point.

NB that this will change the return value for libxl_set_memory_target:
now it will return 0 on success, rather than returning 1 (which was
the previous behavior).  This is more in line with expected behavior,
and also allows the caller to distingiush between success and other
failure modes (some of which also return 1).

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>