- Introduces virNetworkObjGetMetadata() and
  virNetworkObjSetMetadata().
- These functions implement common behaviour that can be reused by
  network drivers that use the virNetworkObj struct.
- Introduces virNetworkObjUpdateModificationImpact() among other
  helper functions that resolve the live/persistent state of
  the network before setting metadata.
- Replace redundant flag resolving code with the forementioned 
  function in bridge_driver.c and test_driver.c 
- Eliminates redundant call of virNetworkObjSetDefTransient() in
  virNetworkConfigChangeSetup().

Signed-off-by: K Shiva Kiran <shiva_kr@riseup.net>
---

This patch is a v2 of:
https://listman.redhat.com/archives/libvir-list/2023-July/240662.html

Diff to v1:
- Replace redundant call of virNetworkObjSetDefTransient() in
  virNetworkConfigChangeSetup()
- Replace redundant virNetworkFlags setting code with
  virNetworkObjUpdateModificationImpact() in bridge_driver.c
  and test_driver.c
- Eliminated newlines in message strings that followed the
  80 character limit.

Shiva