kvm: fix segfault with query-stats-schemas and -M none

kvm: fix segfault with query-stats-schemas and -M none

-M none creates a guest without a vCPU, causing the following error:

$ ./qemu-system-x86_64 -qmp stdio -M none -accel kvm
{execute:qmp_capabilities}
{"return": {}}
{execute: query-stats-schemas}
Segmentation fault (core dumped)

Fix it by not querying the vCPU stats if first_cpu is NULL.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>