This patch modifies the start instance script, opcode and logical unit
to support temporary startup parameters.

Different from 1.2, where only the kernel arguments were supporting
changes (and thus xen-pvm specific), this version supports changing all
hypervisor and backend parameters (with appropriate checks).

This is much more flexible, and allows for example:
  - start with different, temporary kernel
  - start with different memory size

Note: in later versions, this should be extended to cover disk
parameters as well (e.g. start with drbd without flushes, start with
drbd in async mode, etc.).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>