Add -H/-B startup parameters to gnt-instance

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>