devices: vhost-user: block: remove global executor variable
devices: vhost-user: block: remove global executor variable

This variable can be passed as part of the backend structure.

BUG=None
TEST=cargo build

Change-Id: I3c1f6e04417ea2e6c8b2d4d662829cf24b353fdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3450027
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

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