Before now, all the qemu hotplug functions assumed that all devices to
be hotplugged were legacy PCI endpoint devices
(VIR_PCI_CONNECT_TYPE_PCI_DEVICE). This worked out "okay", because all
devices *are* legacy PCI endpoint devices on x86/440fx machinetypes,
and hotplug didn't work properly on machinetypes using PCIe anyway
(hotplugging onto a legacy PCI slot doesn't work, and until commit
b87703cf any attempt to manually specify a PCIe address for a
hotplugged device would be erroneously rejected).

This patch makes all qemu hotplug operations honor the pciConnectFlags
set by the single all-knowing function
qemuDomainDeviceCalculatePCIConnectFlags(). This is done in 3 steps,
but in a single commit since we would have to touch the other points
at each step anyway:

1) add a flags argument to the hypervisor-agnostic
virDomainPCIAddressEnsureAddr() (previously it hardcoded
..._PCI_DEVICE)

2) add a new qemu-specific function qemuDomainEnsurePCIAddress() which
gets the correct pciConnectFlags for the device from
qemuDomainDeviceConnectFlags(), then calls
virDomainPCIAddressEnsureAddr().

3) in qemu_hotplug.c replace all calls to
virDomainPCIAddressEnsureAddr() with calls to
qemuDomainEnsurePCIAddress()

So in effect, we're putting a "shim" on top of all calls to
virDomainPCIAddressEnsureAddr() that sets the right pciConnectFlags.