Dry-run migration

As part of our ongoing work on improving the ease of use of xVM, the newly available build 126 of OpenSolaris has my putback for:

6878952 Would like dry-run migration

This feature is useful for doing a simple check as to whether a guest can successfully migrate to another dom0 host. For example, domu-221 here is using a disk path that doesn't exist on the remote host hiss:

# virsh migrate --dryrun domu-221 xen:/// hiss    
error: POST operation failed: xend_post: error from xen daemon:
(xend.err 'Remote server error: Access to vbd:768 failed: error: "/iscsi/nevada-hvm" is not a valid block device.')

This works both with running and shutdown guests. Currently, the checks are fairly limited: are disks of the same path available on the remote host (note there is no checking of GUIDs or whatever to verify they really are the same piece of shared storage); is there enough memory on the remote host; and is the remote host the same CPU vendor. We expect these checks to improve both in scope and in reliability in the future.