burnin: skip migration based on hypervisor support

The patch adds selection of hypervisor in burnin (on multi-hypervisor
clusters, we always used the default) and optional hypervisor
parameters, and skips migration if the hypervisor doesn't support it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>