vdsm: vm.conf: migrate boot to bootOrder

migrated boot keyword used previously at VM
configuration level to the new specification of
bootOrder now used at devices specification level.

Change-Id: I2f5cb3bde2bcb48d8e943201c3447b572835d5ef
Bug-Url: https://bugzilla.redhat.com/1011031
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>