xen/blkback: do not use x86 CPUID in generic code

Move checker for whether Xen creates IOMMU mappings for foreign pages
into a helper that's defined in arch-specific code.

Reported by: Elliott Mitchell <ehem+freebsd@m5p.com>
Fixes: 1d528f95e8ce ('xen/blkback: remove bounce buffering mode')
Sponsored by: Citrix Systems R&D