KVM: x86: fix incorrect comparison in trace event

[ Upstream commit 147f1a1fe5d7e6b01b8df4d0cbd6f9eaf6b6c73b ]

The "u" field in the event has three states, -1/0/1.  Using u8 however means that
comparison with -1 will always fail, so change to signed char.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>