qemu: Add support to launch an SEV guest

qemu: Add support to launch an SEV guest

QEMU >= 2.12 provides 'sev-guest' object which is used to launch encrypted
VMs on AMD platform using SEV feature. The various inputs required to
launch SEV guest is provided through the <launch-security> tag. A typical
SEV guest launch command line looks like this:

  -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=5 ...\
  -machine memory-encryption=sev0 \

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