Add support for using the bootloader in xen-pvm

This patch adds three optional parameters:
  - 'use_bootloader', whether use or not the bootloader
  - 'bootloader_path', absolute path to the bootloader
  - 'bootloader_args', extra arguments to the bootloader

Syntax:
  gnt-cluster modify --hypervisor-parameters \
    xen-pvm:bootloader_path=/usr/bin/pygrub,use_bootloader=False
  gnt-instance modify -H use_bootloader=True instance1.example.com

If use_bootloader is True, each domU can boot with its own kernel
instead of using the dom0 kernel.

Signed-off-by: Jun Futagawa <jfut@integ.jp>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: ultortter