Add missing HwBreakpointOps impl to GdbStub
Add missing HwBreakpointOps impl to GdbStub

Migration to gdbstub 0.5.0 broke hardware breakpoints.
(https://crrev.com/c/3071900). This fixes it.

BUG=chromium:1259868
TEST=Manually with `hbreak start_kernel` on a volteer device

Change-Id: I35097b4ad2ec9990e78e68c3274c12ba7a5dbea9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3244555
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Woody Chow <cc0bff42e088f24a7e6e350b1e79cd0db9125d07@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm