qemu-options.hx: Update the reduced-phys-bits documentation

qemu-options.hx: Update the reduced-phys-bits documentation

A guest only ever experiences, at most, 1 bit of reduced physical
addressing. Update the documentation to reflect this as well as change
the example value on the reduced-phys-bits option.

Fixes: a9b4942f48 ("target/i386: add Secure Encrypted Virtualization (SEV) object")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <13a62ced1808546c1d398e2025cf85f4c94ae123.1664550870.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>