vmm: move bumping VMEXIT_USERSPACE stat to the right place

Statistic for "number of vm exits handled in userspace" should be
increased in vm_run() instead of vmx_run() because in some cases
vm_run() doesn't exit to userspace and keeps entering the guest.

Also svm_run's implementation even wrongly misses that stat.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35350