Get rid of constants.HT_HVM_DEFAULT_BOOT_ORDER

Confusingly, as a leftober from 1.2, there was a
constants.HT_HVM_DEFAULT_BOOT_ORDER constant, with a value opposite to
the default HV_BOOT_ORDER hv param that got enabled only if
HV_BOOT_ORDER was set to None. Since setting it to None is very
hard/impossible for the user, and we didn't handle other "empty" values
(False, ''), we'll just force the parameter to have a valid value (after
all we have a default, and that's the way we use hvparams) and get rid
of the old constant altoghether.

Reviewed-by: iustinp