undefine: Define the new API

undefine: Define the new API

This introduces a new API virDomainUndefineFlags to control the
domain undefine process, as the existing API virDomainUndefine
doesn't support flags.

Currently only flag VIR_DOMAIN_UNDEFINE_MANAGED_SAVE is supported.
If the domain has a managed save image, including
VIR_DOMAIN_UNDEFINE_MANAGED_SAVE in @flags will also remove that
file, and omitting the flag will cause undefine process to fail.

This patch also changes the behavior of virDomainUndefine, if the
domain has a managed save image, the undefine will be refused.