sev/i386: add support to LAUNCH_MEASURE command

During machine creation we encrypted the guest bios image, the
LAUNCH_MEASURE command can be used to retrieve the measurement of
the encrypted memory region. This measurement is a signature of
the memory contents that can be sent to the guest owner as an
attestation that the memory was encrypted correctly by the firmware.
VM management tools like libvirt can query the measurement using
query-sev-launch-measure QMP command.

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>

target/i386/sev-stub.c

@@ -39,3 +39,8 @@ uint32_t sev_get_reduced_phys_bits(void)
 {
     return 0;
 }
+
+char *sev_get_launch_measurement(void)
+{
+    return NULL;
+}