domain capabilities: Expose firmware auto selection feature

If a management application wants to use firmware auto selection
feature it can't currently know if the libvirtd it's talking to
support is or not. Moreover, it doesn't know which values that
are accepted for the @firmware attribute of <os/> when parsing
will allow successful start of the domain later, i.e. if the mgmt
application wants to use 'bios' whether there exists a FW
descriptor in the system that describes bios.

This commit then adds 'firmware' enum to <os/> element in
<domainCapabilities/> XML like this:

<enum name='firmware'>
<value>bios</value>
<value>efi</value>
</enum>

We can see both 'bios' and 'efi' listed which means that there
are descriptors for both found in the system (matched with the
machine type and architecture reported in the domain capabilities
earlier and not shown here).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>