Attach(vhci) after receiving heart-beat from guest

There seem to be a race between the intial attach to vhci_hcd and
possibly the initialization of virtio-serial and/or usbforwarder. The device
at the root hub (on the host) fails to be connected. The USB host stack fails
to connect (keeps retrying to connect to the device but never succeeds after
this point of failure). Moving from low-speed to high speed certainly provides
one with more deterministic failure. The exact root cause isn't known yet but
moving from low speed to high speed configuration certainly produces more
failure rates. By allowing the Attach to happen after receiving the heart-beat
from the guest, possibly allows virtio-serial initialization and of course the
usbforwarder.

BUG=65292103

Change-Id: I4c19001329c18c021b51b61c31da14a5fd87c684
(cherry picked from commit 3057a97e67c4dc772a1c573fa9d47ecf33dd665b)
1 file changed