i386/sev: Move sev_launch_finish to separate class method

i386/sev: Move sev_launch_finish to separate class method

When sev-snp-guest objects are introduced there will be a number of
differences in how the launch finish is handled compared to the existing
sev-guest object. Move sev_launch_finish() to a class method to make it
easier to implement SNP-specific launch update functionality later.

Signed-off-by: Pankaj Gupta <pankaj.gupta@amd.com>
Message-ID: <20240530111643.1091816-7-pankaj.gupta@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>