accel/kvm: Adjust the note about the minimum required kernel version

accel/kvm: Adjust the note about the minimum required kernel version

Since commit 126e7f78036 ("kvm: require KVM_CAP_IOEVENTFD and
KVM_CAP_IOEVENTFD_ANY_LENGTH") we require at least kernel 4.5 to
be able to use KVM. Adjust the upgrade_note accordingly.
While we're at it, remove the text about kvm-kmod and the
SourceForge URL since this is not actively maintained anymore.

Fixes: 126e7f78036 ("kvm: require KVM_CAP_IOEVENTFD and KVM_CAP_IOEVENTFD_ANY_LENGTH")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit f180e367fce44b336105a11a62edf9610b6b2a06)