pcie_root_port: Add hotplug disabling option

pcie_root_port: Add hotplug disabling option

Make hot-plug/hot-unplug on PCIe Root Ports optional to allow libvirt
manage it and restrict unplug for the whole machine. This is going to
prevent user-initiated unplug in guests (Windows mostly).
Hotplug is enabled by default.
Usage:
    -device pcie-root-port,hotplug=off,...

If you want to disable hot-unplug on some downstream ports of one
switch, disable hot-unplug on PCIe Root Port connected to the upstream
port as well as on the selected downstream ports.

Discussion related:
    https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg00530.html

Signed-off-by: Julia Suvorova <[email protected]>
Message-Id: <20200226174607[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Ján Tomko <[email protected]>