KVM: s390: use defines for execution controls

Let's replace the bitmasks by defines. Reconstructed from code, comments
and commit messages.

Tried to keep the defines short and map them to feature names. In case
they don't completely map to features, keep them in the stye of ICTL
defines.

This effectively drops all "U" from the existing numbers. I think this
should be fine (as similarly done for e.g. ICTL defines).

I am not 100% sure about the ECA_MVPGI and ECA_PROTEXCI bits as they are
always used in pairs.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170313104828.13362-1-david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[some renames, add one missing place]