util: pull gnulib physmem impl into local code

util: pull gnulib physmem impl into local code

We don't need all the platforms gnulib deals with, so
this is a cut down version of GNULIB's physmem.c
code. This also allows us to integrate libvirt's
error reporting functions closer to the error cause.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>