vm: provide a knob to disable computation of the hash

This lets one use RHEL settings with CentOS images, or other similar hacks.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
vm: use normal screendumps to verify BSOD

Currently a BSOD might happen before a screendump is taken, and thus
will not be in the video.  Instead, check the BSOD against the
files that are captured by _take_screendumps.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Merge pull request #1465 from bonzini/pbonzini-small

Two small patches from my queue