scripts/arch-run: Mark migration tests as SKIP if ncat is not available

Instead of failing the tests, we should rather skip them if ncat is
not available.
While we're at it, also mention ncat in the README.md file as a
requirement for the migration tests.

Resolves: https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/issues/4


Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211221092130.444225-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>