qemuxml2xmltest: Redirect access to FW descriptor dirs

qemuxml2xmltest: Redirect access to FW descriptor dirs

If /etc/qemu/firmware directory exists, but is not readable then
qemuxml2xmltest fails. This is because once domain XML is parsed
it is validated. For that domain capabilities are needed.
However, when constructing domain capabilities, FW descriptors
are loaded and this is the point where the test fails, because it
fails to open one of the directories.

Fixes: 5b9819eedc domain capabilities: Expose firmware auto selection feature
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>