conf: Use correct type for balloon stats period

conf: Use correct type for balloon stats period

We're parsing memballoon status period as unsigned int, but when we're
trying to set it, both we and qemu use signed int.  That means large
values will get wrapped around to negative one resulting in error.
Basically the same problem as commit e3a7b874 was dealing with when
updating live domain.

QEMU changed the accepted value to int64 in commit 1f9296b5, but even
values as INT_MAX don't make sense since the value passed means seconds.
Hence adding capability flag for this change isn't worth it.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1140958

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>