xl: initialise rc before using it in vcpuset

xl: initialise rc before using it in vcpuset

In 5b725e56 (xl: improve return and exit codes of vcpu related
functions), the return value of libxl_cpu_bitmap_alloc was not stored in
rc anymore. Yet the subsequent fprintf still used that.

Reinstate the original implementation, that is, to store return value of
libxl_cpu_bitmap_alloc in rc before using rc.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>