docs: virt: user_mode_linux.rst: fix URL references

Several URLs are pointing to outdated places.

Update the references for the URLs whose contents still exists,
removing the others.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>