tests/functional: Add missing require_netdev('user') statements

tests/functional: Add missing require_netdev('user') statements

A bunch of tests are using "-netdev user" but fail to check
for the availability of SLIRP in the binary, so these tests
fail if QEMU has been configured with "--disable-slirp"
(most of the tests are disabled by default with a decorator,
that's likely why nobody noticed this problem yet). Add the
missing self.require_netdev('user') statements to skip the
tests if SLIRP is not available.

Message-ID: <20250324123450.111307-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>