I've tried several existing and new instances of various Ubuntu server instances (12.04 LTS - 13.10). The host is an old desktop I've converted to a host, and is a C2D E6550 with 8GB DDR2.
Basically, if the Ubuntu guests are in an "off" state, and I boot them, they boot normally. The issue is, every time I do a reboot (not a reset), they never come back up. The guest CPU will peg at 100% and never come up. If I turn it off and restart the guest, it comes back up. It's ONLY on reboot. It will get past the GRUB loader and then it sits on a black screen forever, with the CPU pegged.
I have an AMD VM host, 4GB DDR2, and it has no issues rebooting the same instances. I also have a newer Core i7 desktop and it too has no issues rebooting. It's just this E6550 machine.
Windows instances have no issue at all. Just Ubuntu (I've tried Centos but never could get it to boot).
The same machine previously was a Windows 2012 (not R2) server with a Hyper-V role, running the same guests, and don't recall it having this same issue.
Does something in the BIOS need to be adjusted? I have no CPU compatibility modes enabled, and all setups are defaults. Disks are IDE. All OSes are 32-bit.
EDIT: I tried x64 13.10 instance and it works normally. It seems to be only 32-bit. I was using 32-bit mainly due to the small amount of RAM required/needed for these instances.