devices: virtio: vhost: user: Refactor BlockBackend::new
devices: virtio: vhost: user: Refactor BlockBackend::new

BUG=b:194137301
TEST=run 'crosvm device block ...'

Change-Id: I378dfbc95ee941cdd86aad3c1c42fcdad2e26d0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3149875
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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