qemu: Remove support for legacy block jobs

qemu: Remove support for legacy block jobs

Block job QMP commands with underscores rather than dashes were never
released in upstream qemu, (they were added, but modified in the same
release [1]), but a certain distro managed to backport the version in the
middle.

The change also slightly modified semantics for the abort command, which
made us have a lot of code which was only ever present in certain
downstream distros.

Clean the upstream code from the legacy cruft and support only the
upstream implementations.

[1] See qemu commit v1.0-2176-gdb58f9c060

Reviewed-by: Eric Blake <eblake@redhat.com>