KVM: x86: flush TLB separately from MMU reset

For both CR0 and CR4, disassociate the TLB flush logic from the
MMU role logic.  Instead  of relying on kvm_mmu_reset_context() being
a superset of various TLB flushes (which is not necessarily going to
be the case in the future), always call it if the role changes
but also set the various TLB flush requests according to what is
in the manual.

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