update-linux-headers: copy standard-headers files one by one

update-linux-headers: copy standard-headers files one by one

cp_virtio is called for both the asm-s390/ and linux/ directories,
so it looks for pci_regs.h and input.h files in asm-s390/ too.  This
makes little sense.  In the next patch we will have the opposite
problem; we want to add asm-x86/hyperv.h, and there's also a
linux/hyperv.h file with unwanted dependencies on additional Linux
uapi headers.  We do not want to copy linux/hyperv.h.

The solution is to make cp_virtio (now renamed to cp_portable) copy
one file only, instead of using the "find" command, and call it multiple
times.  The new function is really just a reindentation of the old one.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>