core: Fix CPU shares in VmInfoBuilder

When CPU shares are 0, the info builder should not include the cpuShares
node in the info (so that in vdsm the CPU shares capability will just be disabled)

Change-Id: I45e75b2c91049f5e7e74e208adcdcf0ee7c3a8d6
Signed-off-by: Ofri Masad <omasad@redhat.com>