devices: virtio: pci: reset ISR status on read
devices: virtio: pci: reset ISR status on read

The virtio PCI spec (4.1.4.5.1 Device Requirements: ISR status
capability) says:

  "The device MUST reset ISR status to 0 on driver read."

BUG=chromium:854766
TEST=None

Change-Id: I92a1ddccfc8e44bed7f4a16e3cfd11b946629e22
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1260252
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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