bridge_driver: Introduce a helper for virNetworkObjSetMacMap()

bridge_driver: Introduce a helper for virNetworkObjSetMacMap()

Currently, whenever virNetworkObjSetMacMap() is called the same
pattern is used:

1) call virMacMapFileName() to generate a filename,
2) pass this filename to virMacMapNew(), and finally
3) pass retval from previous step to virNetworkObjSetMacMap().

Move this code into a helper (networkSetMacMap()) and replace
both pattern occurrences with its call.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>