Remove the pointer from def->cputune.vcpupin after unplugging
the CPU and also free the bitmap contained in the structure
by calling virDomainVcpuPinDel instead of VIR_FREE.

Introduced by commit 0df1a790.

This makes virDomainLookupVcpuPin redundant.

https://bugzilla.redhat.com/show_bug.cgi?id=1088165