Allow an arbitrary timezone with QEMU by setting the $TZ environment
variable when launching QEMU
* src/qemu/qemu_conf.c: Set TZ environment variable if a timezone
is requested
* tests/qemuxml2argvtest.c: Add test case for timezones
* tests/qemuxml2argvdata/qemuxml2argv-clock-france.xml,
tests/qemuxml2argvdata/qemuxml2argv-clock-france.args: Data
for timezone tests
