This patch adds functions for modify domain's persistent definition.
To do error recovery in easy way, we use a copy of vmdef and update it.

The whole sequence will be:

  make a copy of domain definition.

  if (flags & MODIFY_CONFIG)
      update copied domain definition
  if (flags & MODIF_LIVE)
      do hotplug.
  if (no error)
      save copied one to the file and update cached definition.
  else
      discard copied definition.

This patch is mixuture of Eric Blake's work and mine.
From: Eric Blake <eblake@redhat.com>
Signed-off-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

(virDomainObjCopyPersistentDef): make a copy of persistent vm definition
(qemuDomainAttach/Detach/UpdateDeviceConfig) : callbacks. now empty
(qemuDomainModifyDeviceFlags): add support for MODIFY_CONFIG and MODIFY_CURRENT