xen-mapcache.c contains common functions which can be used for enabling Xen on
aarch64 with IOREQ handling. Moving it out from hw/i386/xen to hw/xen to make it
accessible for both aarch64 and x86.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Paul Durrant <paul@xen.org>