cutils: add qemu_strtoi & qemu_strtoui parsers for int/unsigned int types

There are qemu_strtoNN functions for various sized integers. This adds two
more for plain int & unsigned int types, with suitable range checking.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>