Updated the list of network types that are checked prior to network
deletion. This disallows pxeboot type network to be deleted after
initial system config.
Enable the unit test to ensure semantic check forbids pxeboot network
delete.
Closes-Bug: 1859855
Change-Id: I9a9d25abadc1e4f0cade2a4efef3c8adf3f0262b
Signed-off-by: Thomas Gao <Thomas.Gao@windriver.com>