x86/xen: Use intended values in HVM_CALLBACK_VECTOR

Replace the HVM_CB_GSI_GSI SHIFT and MASK with the existing
but currently unused HVM_CB_VECTOR_VECTOR_SHIFT and MASK.

The binary values are identical.

Reviewed by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Pull request: https://github.com/freebsd/freebsd-src/pull/1490