KVM: MMU: make kvm_mmu_reset_context() flush the guest TLB

kvm_set_cr0() and kvm_set_cr4(), and possible other functions,
assume that kvm_mmu_reset_context() flushes the guest TLB.  However,
it does not.

Fix by flushing the tlb (and syncing the new root as well).

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