KVM: x86: move steal time initialization to vcpu entry time

As reported at https://bugs.launchpad.net/qemu/+bug/1494350

,
it is possible to have vcpu->arch.st.last_steal initialized
from a thread other than vcpu thread, say the iothread, via
KVM_SET_MSRS.

Which can cause an overflow later (when subtracting from vcpu threads
sched_info.run_delay).

To avoid that, move steal time accumulation to vcpu entry time,
before copying steal time data to guest.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>