Convert virDomainVcpuPinDefPtr to virDomainPinDefPtr

Convert virDomainVcpuPinDefPtr to virDomainPinDefPtr

As pointed out by jtomko in his review of the IOThreads pinning code:

http://www.redhat.com/archives/libvir-list/2015-March/msg00495.html

there are some comments sprinkled in indicating IOThreads were using
the same structure as the VcpuPin code...

This is the first patch of a few that will change the virDomainVcpuPin*
structures and code to just virDomainPin* - starting with the data
structure naming...