KVM: VMX: do not overwrite uptodate vcpu->arch.cr3 on KVM_SET_SREGS

Only decache guest CR3 value if vcpu->arch.cr3 is stale.
Fixes loadvm with live guest.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Tested-by: Markus Schade <markus.schade@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>