target-arm: kvm - re-inject guest debug exceptions

target-arm: kvm - re-inject guest debug exceptions

If we can't find details for the debug exception in our debug state
then we can assume the exception is due to debugging inside the guest.
To inject the exception into the guest state we re-use the TCG exception
code (do_interrupt).

However while guest debugging is in effect we currently can't handle the
guest using single step as we will keep trapping to back to userspace.
GDB makes heavy use of single-step behind the scenes which effectively
means the guest's ability to debug itself is disabled while it is being
debugged.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1449599553-24713-6-git-send-email-alex.bennee@linaro.org
[PMM: Fixed a few typos in comments and commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>