xen/pciback: Move the PCI_DEV_FLAGS_ASSIGNED ops to the "[un|]bind"

operation instead of doing it per guest creation/disconnection. Without
this we could have potentially unloaded the vf driver from the
xen pciback control even if the driver was binded to the xen-pciback.
This will hold on to it until the user "unbind"s the PCI device using
SysFS.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>