qemu: do upfront check for vcpupids being null when querying pinning

qemu: do upfront check for vcpupids being null when querying pinning

The qemuDomainHelperGetVcpus attempted to report an error when the
vcpupids info was NULL. Unfortunately earlier code would clamp the
value of 'maxinfo' to 0 when nvcpupids was 0, so the error reporting
would end up being skipped.

This lead to 'virsh vcpuinfo <dom>' just returning an empty list
instead of giving the user a clear error.

(cherry picked from commit 9358b63a0dfff6bc5a62698ea45d4fb5db91fa47)