The existing xc_domain_dumpcore is very specific to disk/file based

The existing xc_domain_dumpcore is very specific to disk/file based
output.  Refactor the code slightly to allow more user-specified
control.  This is done by adding a parallel
xc_domain_dumpcore_via_callback, which allows the specification of a
callback routine and an opaque argument block.  The existing dumpcore
routine is modified to use the callback for all write operations and
to turn the single seek into a small write (it's for page alignment).

Signed-off-by: Ben Thomas <bthomas@virtualiron.com>