KVM: ARM: Handle guest faults in KVM

Handles the guest faults in KVM by mapping in corresponding user pages
in the 2nd stage page tables.

We invalidate the instruction cache by MVA whenever we map a page to the
guest (no, we cannot only do it when we have an iabt because the guest
may happily read/write a page before hitting the icache) if the hardware
uses VIPT or PIPT.  In the latter case, we can invalidate only that
physical page.  In the first case, all bets are off and we simply must
invalidate the whole affair.  Not that VIVT icaches are tagged with
vmids, and we are out of the woods on that one.  Alexander Graf was nice
enough to remind us of this massive pain.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>