libvmmapi: Move PCI passthrough ioctl wrappers into a separate file

The arm64 port doesn't implement PCI passthrough and in particular
doesn't define the ioctls used by these wrappers. It might be that the
ppt ioctl interface will require modification to support arm64. Until
that's sorted out one way or another, put this code in a separate file
so that it's easy to conditionally compile.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41003