kvm: x86: Allow to respond to generic signals during slow PF

Enable x86 slow page faults to be able to respond to non-fatal signals,
returning -EINTR properly when it happens.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221011195947.557281-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>