Clarify error message when a PCI slot is already in use (v2)

Clarify error message when a PCI slot is already in use (v2)

When mistakenly configuring two devices in the same PCI slot,
QEMU gives a not entirely obvious message about a 'devfn' being
in use:

$ qemu -device rtl8139 -device virtio-balloon-pci,bus=pci.0,addr=0x3
qemu-kvm: -device virtio-balloon-pci,bus=pci.0,addr=0x3: PCI: devfn 24 not available for virtio-balloon-pci, in use by rtl8139

The user does not configure 'devfn' numbers, they use slot+function.
Thus the error messages should be reported back to the user with that
same terminology rather than the internal QEMU terminology. This
patch makes it report:

$ qemu -device rtl8139 -device virtio-balloon-pci,bus=pci.0,addr=0x3
qemu: -device virtio-balloon-pci,bus=pci.0,addr=0x3.7: PCI: slot 3 function 0 not available for virtio-balloon-pci, in use by rtl8139

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>