kvm: selftests: ensure vcpu file is released

The selftests were not munmap-ing the kvm_run area from the vcpu file descriptor.
The result was that kvm_vcpu_release was not called and a reference was left in the
parent "struct kvm".  Ultimately this was visible in the upcoming state save/restore
test as an error when KVM attempted to create a duplicate debugfs entry.

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