xenstat: fix unsigned less-than-0 comparison

xenstat: fix unsigned less-than-0 comparison

Commit 1438d36f ("xenstat: Fix buffer over-run with new_domains being
negative.") attempted to fix the handling of a negative error result
from xc_domain_getinfolist in xenstat_get_node. However, it forgot to
change the result variable from an unsigned type to a signed one.

Do so, allowing the error result to be handled properly.

Signed-off-by: Matthew Daley <mattd@bugfuzz.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>