qemu: new capabilities flag pcie-root-port.hotplug

qemu: new capabilities flag pcie-root-port.hotplug

This caps flag is set when the qemu binary supports the option
"hotplug" for pcie-root-port, ioh3420 (Intel pcie-root-port) and
xio3130-downstream (Intel pcie-downstream-port). If it's available,
it's possible to disable hotplugging/unplugging devices on a
particular port by adding ",hotplug=off" to the qemu device
commandline. This option first appears in qemu-5.0.0.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>