libelf: avoid UB in elf_xen_feature_{get,set}()

libelf: avoid UB in elf_xen_feature_{get,set}()

When the left shift amount is up to 31, the shifted quantity wants to be
of unsigned int (or wider) type.

While there also adjust types: get doesn't alter the array and returns a
boolean, while both don't really accept negative "nr". Drop a stray
blank each as well.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@gmail.com>