virtio_pci_device: Move device activation code to `activate()`
virtio_pci_device: Move device activation code to `activate()`

Increase readability by reducing nesting and function length.
Also, remove Option from VirtioPciDevice.mem as it cannot be
None.

BUG=b:215307964
TEST=cargo test

Change-Id: I66d57d5205a59a507433d71de71ca58a98cfeb6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3460790
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Woody Chow <cc0bff42e088f24a7e6e350b1e79cd0db9125d07@google.com>

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