nvme: Fix hotplug on one of the amazon platformsHEADmain

Amazon EC2 m7i cloud instances use PCI hotplug rather than ACPI
hotplug. The card is removed and detach is called to remove the drive
from the system. The hardware is no longer present at this point, but
the bridge doesn't translate the now-missing hardware reads to all ff's
leading us to conclude the hardware is there and we need to do a proper
shutdown of it. Fix this oversight by asking the bridge if the device is
still present as well. We need both tests since some systems one cane
remove the card w/o a hotplug event and we want to fail-safe in those
cases.

Convert gone to a bool while I'm here and update a comment about
shutting down the controller and why that's important.

Tested by: cperciva
Sponsored by: Netflix

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