The final patch in Hu Tao's series to enhance virBitmap actually
removes virDomainCpuSetParse and virDomainCpuSetFormat as "no longer
used", and the rest of the series hadn't taken care of two uses of
virDomainCpuSetParse in the xen code.

This patch replaces those with appropriate virBitmap functions. It
should be pushed prior to the patch removing virDomainCpuSetParse.