docs: kvm: fix WARNINGs from api.rst

Use the api number 134 for KVM_GET_XSAVE2, instead of 42, which has been
used by KVM_GET_XSAVE.
Also, fix the WARNINGs of the underlines being too short.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Message-Id: <20220120045003.315177-1-wei.w.wang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>