qemu_firmware: Introduce qemuFirmwareFetchConfigs

qemu_firmware: Introduce qemuFirmwareFetchConfigs

Implementation for yet another part of firmware description
specification. This one covers selecting which files to parse.

There are three locations from which description files can be
loaded. In order of preference, from most generic to most
specific these are:

  /usr/share/qemu/firmware
  /etc/qemu/firmware
  $XDG_CONFIG_HOME/qemu/firmware

If a file is found in two or more locations then the most specific
one is used. Moreover, if file is empty then it means it is
overriding some generic description and disabling it.

Again, this is described in more details and with nice examples
in firmware.json specification (qemu commit 3a0adfc9bf).

However, there's one slight difference - for the root user the
home directory is not searched. This follows rules laid out by
similar look up processes, e.g. PKI x509 certs are not searched
in /root but they are looked for under /home.

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