drm/i915/gvt: Get reference to KVM iff attachment to VM is successful

Get a reference to KVM if and only if a vGPU is successfully attached to
the VM to avoid leaking a reference if there's no available vGPU.  On
open_device() failure, vfio_device_open() doesn't invoke close_device().

Fixes: 421cfe65 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
Cc: stable@vger.kernel.org
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20221111002225.2418386-2-seanjc@google.com