conf: Introduce launch-security element in domain

conf: Introduce launch-security element in domain

The launch-security element can be used to define the security
model to use when launching a domain. Currently we support 'sev'.

When 'sev' is used, the VM will be launched with AMD SEV feature enabled.
SEV feature supports running encrypted VM under the control of KVM.
Encrypted VMs have their pages (code and data) secured such that only the
guest itself has access to the unencrypted version. Each encrypted VM is
associated with a unique encryption key; if its data is accessed to a
different entity using a different key the encrypted guests data will be
incorrectly decrypted, leading to unintelligible data.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>