KVM: MMU: init kvm generation close to mmio wrap-around value

Then it has the chance to trigger mmio generation number wrap-around

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
[Change from MMIO_MAX_GEN - 13 to MMIO_MAX_GEN - 150, because 13 is
 very close to the number of calls to KVM_SET_USER_MEMORY_REGION
 before the guest is started and there is any chance to create any
 spte. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>