devices: pci: msix: lazy MSI vector allocation
devices: pci: msix: lazy MSI vector allocation

If device does not use all of the MSI vectors, we do not need to
register all for them. This patch pushes the MSI vector registery
to the first time they are enabled.

BUG=None
TEST=./tools/presubmit

Change-Id: I5453e93af4f4f5596d82ad126dc294fa5896846a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3490236
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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