There should be at least one domain for each guest
in cababilities. And in current code we don't add
domain for this guest for example.

    if ((guest = virCapabilitiesAddGuest(caps, VIR_DOMAIN_OSTYPE_HVM,
                                         VIR_ARCH_X86_64,
                                         "vz",
                                         NULL, 0, NULL)) == NULL)

Anyway, with two virt types it looks a litte messy, so let's
move adding guest and domain to a separate function.
Signed-off-by: NDmitry Guryanov <dguryanov@parallels.com>