KVM: MIPS: let generic code call prepare_flush_shadow

Since all calls to kvm_flush_remote_tlbs must be preceded by
kvm_mips_callbacks->prepare_flush_shadow, repurpose
kvm_arch_flush_remote_tlb to invoke it.  This makes it possible
to use the TLB flushing mechanism provided by the generic MMU
notifier code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>