host-utils: Simplify pow2ceil()

Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1501148776-16890-4-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>