core, engine: Fix balloon detection during VM import

The canDoAction method of the ImportVmCommand was not properly checking
for the presence of ballooning devices and always blocked the importing
of ppc64 VMs. This patch fixes this issue.

Change-Id: If660e368d7ec31ff1f8251d483971ffbf0bd858a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1152560
Signed-off-by: Vitor de Lima <vdelima@redhat.com>