conf: Report default hyperv values in domain capabilities

conf: Report default hyperv values in domain capabilities

So far the set of available Hyper-V enlightenments are reported
in domain capabilities. Well, some enlightenments are more than
just simple on/off switch. For instance, the 'spinlocks'
enlightenment expects a number, or 'vendor_id' expects a string.

All of these have some default values (at least in QEMU) and are
used when the passthrough mode is set.

Allow querying these defaults in domain capabilities XML.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>