virsh: fix broken code in freepages

virsh: fix broken code in freepages

Commit 9e3efe53 broke the build under valgrind or clang, by writing
8 bytes through an allocation of 4 bytes.  It also risks multiplication
overflow when mallocing (that's a pervasive problem that needs an
audit in the rest of the code, but we might as well fix this one while
we are here), and had a typo.

* tools/virsh-host.c (cmdFreepages): Avoid integer overflow and
undefined behavior.

Signed-off-by: Eric Blake <eblake@redhat.com>