target/i386: add Secure Encrypted Virtualization (SEV) object

Add a new memory encryption object 'sev-guest'. The object will be used
to create encrypted VMs on AMD EPYC CPU. The object provides the properties
to pass guest owner's public Diffie-hellman key, guest policy and session
information required to create the memory encryption context within the
SEV firmware.

e.g to launch SEV guest
 # $QEMU \
    -object sev-guest,id=sev0 \
    -machine ....,memory-encryption=sev0

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>