Support hot insertion and removal of PCI devices on EC2.

Install ACPI notify handlers on PCI devices with an _EJ0 method. This
handler is invoked when devices are added or removed.

• When an ACPI_NOTIFY_DEVICE_CHECK event posts, rescan the parent bus device. Note that strictly speaking we only need to rescan the specified device, but BUS_RESCAN is what is available, so we rescan the entire bus.
• When an ACPI_NOTIFY_EJECT_REQUEST event posts, detach the device associated with the ACPI handle, invoke the _EJ0 method, and then delete the device.

Eventually this might be changed to vector notify events to devd in
userspace where devctl can be used instead to permit more complex
actions such as graceful unmounting of filesystems.

Tested by: cperciva
Reviewed by: cperciva, imp, scottl
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21948