KVM: x86: Convert MSR_KVM_SYSTEM_TIME to use gfn_to_hva_cache functions (CVE-2013-1797)

There is a potential use after free issue with the handling of
MSR_KVM_SYSTEM_TIME.  If the guest specifies a GPA in a movable or removable
memory such as frame buffers then KVM might continue to write to that
address even after it's removed via KVM_SET_USER_MEMORY_REGION.  KVM pins
the page in memory so it's unlikely to cause an issue, but if the user
space component re-purposes the memory previously used for the guest, then
the guest will be able to corrupt that memory.

Tested: Tested against kvmclock unit test

Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
static void update_eoi_exitmap(struct kvm_vcpu *vcpu)
{
	u64 eoi_exit_bitmap[4];

	memset(eoi_exit_bitmap, 0, 32);

	kvm_ioapic_calculate_eoi_exitmap(vcpu, eoi_exit_bitmap);
	kvm_x86_ops->load_eoi_exitmap(vcpu, eoi_exit_bitmap);
}

static void update_eoi_exitmap(struct kvm_vcpu *vcpu)
{
	u64 eoi_exit_bitmap[4];

	memset(eoi_exit_bitmap, 0, 32);

	kvm_ioapic_calculate_eoi_exitmap(vcpu, eoi_exit_bitmap);
	kvm_x86_ops->load_eoi_exitmap(vcpu, eoi_exit_bitmap);
}
