Only respond to the PCIe Attention Button if a device is already plugged in.

Prior to this commit, if PCIEM_SLOT_STA_ABP and PCIEM_SLOT_STA_PDC are
asserted simultaneously, FreeBSD sets a 5 second "hardware going away" timer
and then processes the "presence detect" change. In the (physically
challenging) case that someone presses the "attention button" and inserts
a new PCIe device at exactly the same moment, this results in FreeBSD
recognizing that the device is present, attaching it, and then detaching it
5 seconds later.

On EC2 "bare metal" hardware this is the precise sequence of events which
takes place when a new EBS volume is attached; virtual machines have no
difficulty effecting physically implausible simultaneity.

This patch changes the handling of PCIEM_SLOT_STA_ABP to only detach a
device if the presence of a device was detected *before* the interrupt
which reports the Attention Button push.

Reported by: Matt Wilson
Reviewed by: jhb
MFC after: 1 week
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D20499