Fix regression introduced by -machine accel=

Commit 85097db6 changed the timing when kvm_allowed is set until after
kvm is initialized.  During initialization, the ioeventfd initialization code
checks kvm_enabled() and after this change, ioeventfd is effectively disabled.

This causes a significant regression in performance.

Fix this by setting kvm_allowed before calling init.

Reported-by: Khoa Huynh <khoa@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
