KVM: x86: extract blocking logic from __vcpu_run

Rename the old __vcpu_run to vcpu_run, and extract part of it to a new
function vcpu_block.

The next patch will add a new condition in vcpu_block, avoid extra
indentation.

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