virttest.qemu_devices: Add parameter to workaround qemu qmp crash bug

In order to avoid qemu crash dump on older qemu you can set
"workaround_qemu_qmp_crash = always" in base.cfg and qemu_devices
will wait 1 second before using QMP monitor for the first time.

By default virt-test will try using QMP directly after qemu startup
and in case of failure it still fall backs to the workaround. Still
the first attempt generates unnecessarily crashdump which might
be stored in the autotest results.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>