target/i386: SEV: Refactor check_sev_features()

Refactor check_sev_features() to consolidate SEV-SNP checks to a single
if block. This is also helpful when adding checks for future SEV
features.  While at it, move the comment about the checks being done
outside of the function body and expand it to describe what this
function does. Update error_setg() invocations to use a consistent
format.

No functional change intended.

Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/cae04d88adfbcdc2997e518475f3b89091adf8a9.1779281646.git.naveen@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>