MCE: make oops and vmcore MCEs a bit more similar

For oops-MCEs, change abrt-action-check-oops-for-hw-error
to write MCE description to "backtrace" instead of "comment"
element.

Extend abrt-action-check-oops-for-hw-error to be able to
detect vmcore MCEs: it will create "not-reportable" element
for them too, as it was doing it for oops-MCEs.

Add post-create processing to vmcore_event.conf which
runs abrt-action-check-oops-for-hw-error on vmcores.
Since that needs dmesg, dmesg extraction also moved
to this event from analyze_VMcore.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>