sev/i386: add command to create launch memory encryption context

sev/i386: add command to create launch memory encryption context

The KVM_SEV_LAUNCH_START command creates a new VM encryption key (VEK).
The encryption key created with the command will be used for encrypting
the bootstrap images (such as guest bios).

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>