KVM: arm: guest debug, add stub KVM_SET_GUEST_DEBUG ioctl

This commit adds a stub function to support the KVM_SET_GUEST_DEBUG
ioctl. Any unsupported flag will return -EINVAL. For now, only
KVM_GUESTDBG_ENABLE is supported, although it won't have any effects.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org&gt;.>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>