selftests: kvm: Add vm_get_fd() in kvm_util

Introduces the vm_get_fd() function in kvm_util which returns
the VM file descriptor.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200410231707.7128-8-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>