conf: Don't default to raw format for loader/NVRAM

conf: Don't default to raw format for loader/NVRAM

Due to the way the information is stored by the XML parser, we've
had this quirk where specifying any information about the loader
or NVRAM would implicitly set its format to raw. That is,

  <nvram>/path/to/guest_VARS.fd</nvram>

would effectively be interpreted as

  <nvram format='raw'>/path/to/guest_VARS.fd</nvram>

forcing the use of raw format firmware even when qcow2 format
would normally be preferred based on the ordering of firmware
descriptors. This behavior can be worked around in a number of
ways, but it's fairly unintuitive.

In order to remove this quirk, move the selection of the default
firmware format from the parser down to the individual drivers.

Most drivers only support raw firmware images, so they can
unconditionally set the format early and be done with it; the
QEMU driver, however, supports multiple formats and so in that
case we want this default to be applied as late as possible,
when we have already ruled out the possibility of using qcow2
formatted firmware images.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>