QEMU 2.7 and newer don't allow guests to start unless the initial
vCPUs count is a multiple of the vCPU hotplug granularity, so
validate it and report an error if needed.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1283700Signed-off-by: NAndrea Bolognani <abologna@redhat.com>