vz: factor out config update flags checks

vz: factor out config update flags checks

Actually this is not pure refactoring. Part of common code is
replaced with virDomainObjUpdateModificationImpact and this
a good replacement. It includes removed check of inactive
domain and active flags set. Additionally we resolve
current flag in accordance with current state of domain.
Thus it becames possible to attach/detach devices for
inactive domains if this flag is set.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>