vm: update only network QDB entries on netvm change

Calling create_qdb_entries() on netvm change fires domain-qdb-create
which rewrites all QubesDB entries including /keyboard-layout, resetting
any custom layout set by the user in the AppVM.

Add update_qdb_netvm_entries() in QubesVM that writes only the
network-related QubesDB entries (IP, gateway, netmask, DNS, MAC).
Use it in create_qdb_entries() to avoid code duplication, and call it
from on_property_set_netvm() instead of create_qdb_entries().

Update tests to mock update_qdb_netvm_entries() instead of
create_qdb_entries() where netvm changes are tested.

Fixes QubesOS/qubes-issues#9892

Signed-off-by: Nancy <9d.24.nancy.sangani@gmail.com>

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Author: nancysangani