KVM: selftests: Make the number of vcpus global

We also check the input number of vcpus against the maximum supported.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20201104212357.171559-8-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>