selftests: kvm: add set_boot_cpu_id test

Test for the KVM_SET_BOOT_CPU_ID ioctl.
Check that it correctly allows to change the BSP vcpu.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210318151624.490861-2-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>