libxl: attach PCI device to qemu only after setting pciback/pcifront

libxl: attach PCI device to qemu only after setting pciback/pcifront

When qemu is running in stubdomain, handling "pci-ins" command will fail
if pcifront is not initialized already. Fix this by sending such command
only after confirming that pciback/front is running.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>