This adds support for SW breakpoints inserted by userspace.

We do this by trapping all guest software debug exceptions to the
hypervisor (MDCR_EL2.TDE). The exit handler sets an exit reason of
KVM_EXIT_DEBUG with the kvm_debug_exit_arch structure holding the
exception syndrome information.

It will be up to userspace to extract the PC (via GET_ONE_REG) and
determine if the debug event was for a breakpoint it inserted. If not
userspace will need to re-inject the correct exception restart the
hypervisor to deliver the debug exception to the guest.

Any other guest software debug exception (e.g. single step or HW
assisted breakpoints) will cause an error and the VM to be killed. This
is addressed by later patches which add support for the other debug
types.

Signed-off-by: Alex Benn?e <redacted>
Reviewed-by: Christoffer Dall <redacted>

---
v2
  - update to use new exit struct
  - tweak for C setup
  - do our setup in debug_setup/clear code
  - fixed up comments
v3:
  - fix spacing in KVM_GUESTDBG_VALID_MASK
  - fix and clarify wording on kvm_handle_guest_debug
  - handle error case in kvm_handle_guest_debug
  - re-word the commit message
v4
  - rm else leg
  - add r-b-tag

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index ba635c7..33c8143 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt