tools/libxc: Fix error checking for xc_get_{cpu, node}map_size() callers

tools/libxc: Fix error checking for xc_get_{cpu, node}map_size() callers

c/s 2e82c18cd850592ae9a1f682eb93965a868b5f2f changed the error returns of
xc_get_{cpu,node}map_size() to now include returning -1.  This invalidated the
error checks from callers, which expected 0 to be the only error case.

Coverity ID: 1135907 1135908 1135909 1135910 1135911 1135912 1135913 1135914

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>