block/nfs: bump libnfs requirement to 1.9.3

libnfs prior to 1.9.3 contains a bug that will report
wrong transfer sizes if the file offset grows beyond 4GB
and RPC responses are received out of order. this
error is not detectable and fixable in qemu.

additionally 1.9.3 introduces support for handling short
read/writes in general and takes care of the necessary
retransmissions internally.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

configure

@@ -3868,7 +3868,7 @@ fi
 ##########################################
 # Do we have libnfs
 if test "$libnfs" != "no" ; then
-  if $pkg_config --atleast-version=1.9.2 libnfs; then
+  if $pkg_config --atleast-version=1.9.3 libnfs; then
     libnfs="yes"
     libnfs_libs=$($pkg_config --libs libnfs)
     LIBS="$LIBS $libnfs_libs"