xen: add a Xen to BSD error translation

Since Xen and FreeBSD error codes are completely different add a
translation layer in order to convert Xen error codes into native
FreeBSD error codes. This will be used by the privcmd device, which
needs to return the hypercall errors into user-space.

Sponsored by: Citrix Systems R&D

xen/error.h:

• Import Xen error codes.
• Create a table to map Xen error codes into FreeBSD native error codes.
• Create an inline function that performs the translation.