KVM: SVM: reset mmu context in init_vmcb

Since commit aad82703 no mmu reinitialization is performed
via init_vmcb.

Zero vcpu->arch.cr0 and pass the reset value as a parameter to
kvm_set_cr0.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>