Seems like I'm not the only one with this problem on 12.04.
I have set the virtual machine to use 1 CPU and disabled IO APIC.
This is from a previous crash (taken from /var/log/syslog):
Jul 13 10:30:13 linuxerio kernel: [ 241.693372] warning: `VirtualBox' uses 32-bit capabilities (legacy support in use)
Jul 13 10:31:51 linuxerio kernel: [ 339.571390] EXT4-fs (sda6): Unaligned AIO/DIO on inode 5377259 by VirtualBox; performance will be poor.
Thanks in advance.