arm64/vmm: Don't set MDSCR_KDE in the guest when single stepping

[markj]

Summary

This is unneeded, and causes the host to freeze when VHE is enabled.
In particular, when MDSCR_EL2.KDE is set to 1 and the CPU is executing
at EL_D, i.e., EL2, debug exceptions are enabled. In non-VHE mode, we
call into the guest by trapping to EL2, which implicitly masks debug
exceptions by setting PSTATE.D. However, in VHE mode, PSTATE.D remains
clear, so when the guest's MDSCR_EL1 value is loaded, we immediately
begin single-stepping.

In non-VHE mode there is no need to set KDE either, so just stop setting
it.

[andrew]

Do we need it for non-VHE? If so we will need to selectively set it based on in_vhe().

[markj]

In D48965#1117158, @andrew wrote:

Do we need it for non-VHE? If so we will need to selectively set it based on in_vhe().

It's not required - see the updated review description.