qapi, i386/sev: Change the reduced-phys-bits value from 5 to 1

A guest only ever experiences, at most, 1 bit of reduced physical
addressing. Change the query-sev-capabilities json comment to use 1.

Fixes: 31dd67f6 ("sev/i386: qmp: add query-sev-capabilities command")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <cb96d8e09154533af4b4e6988469bc0b32390b65.1664550870.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>