tools/hv: Check for read/write errors

commit 436473bc2173499ae274d0f50111d1e355006caf upstream.

hv_kvp_daemon currently does not check whether fread() or fwrite()
succeed.  Add the necessary checks.  Also, remove the incorrect use of
feof() before fread().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>