qemu: Rewrite qemuFirmwareFetchParsedConfigs()

qemu: Rewrite qemuFirmwareFetchParsedConfigs()

Instead of returning the list of paths exactly as obtained
from qemuFirmwareFetchConfigs(), and allocating the list of
firmwares to be exactly that size right away, start with two
empty lists and add elements to them one by one.

At the moment this only makes things more verbose, but later
we're going to change things so that it's possible that some
of the paths/firmwares are not included in the lists returned
to the caller, and at that point the changes will pay off.

Note that we can't use g_auto() for the new list of paths,
because until the very last moment it's not null-terminated,
so g_strfreev() wouldn't be able to handle it correctly.

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