I try to boot a XEN VM and it looks like the hard drive naming changed from
/dev/hda1. So the boot process drops me to a busy box in initramfs. How can I specify the probably right boot device on the fly and continue the booting?
If you're asking how to boot this one time, usually you just mount the filesystems to their correct locations, then exit and the boot process will continue.
If you want to fix this permanently you can change your fstab file to use drive labels.