virtio-balloon: Tweak recent fix for integer overflow

Commit 1f9296b avoids "other kinds of overflow" by limiting the
polling interval to UINT_MAX.  The computations to protect are done in
64 bits.  This is indeed safe when unsigned is 32 bits, as it commonly
is.  It isn't when unsigned is 64 bits.  Purely theoretical; I'm not
aware of such a system.  Limit it to UINT32_MAX instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
