libxl: Use proper path for 'do_flr' functionality.

libxl: Use proper path for 'do_flr' functionality.

Commit f74035d3b1e827ff6ff1873e2f10feb011a8d0d2 "xl: PCI code cleanups"
introduced an regression where it changed the name from:
/sys/bus/pci/drivers/pciback/do_flr
to
libxl_sprintf(ctx, "%s/pciback/do_flr", SYSFS_PCI_DEV);

And SYSFS_PCI_DEV is "/sys/bus/pci/devices", meaning we would
do /sys/bus/pci/devices/pciback/do_flr. The proper define
should have been SYSFS_PCIBACK_DRIVER, which is what this patch does.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>