virCapabilitiesAddGuestDomain() takes an optional binary
name: this is intended for cases where a certain domain
type can't use the default one registered for the guest
architecture, but has to use a special binary instead.

The current code, however, will pass 'binary' again when
'kvmbin' is not defined, which is unnecessary as 'binary'
has been registered as default earlier, and will result
in capabilities output such as

  <emulator>/usr/bin/qemu-system-x86_64</emulator>
  <domain type='qemu'/>
  <domain type='kvm'>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
  </domain>

with the second <emulator> element providing no additional
information.

Change it so that, when 'kvmbin' is not defined, NULL is
passed and so the default emulator will be used instead.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>