We want to provide to Qemu extra flags such as extra '-serial'
parameter. If we extend vQEMU_FLAGS then serial device goes to wrong
UART since first '-serial ${iQEMU_CONSOLE}' is already defined. The
order of UARTs does matter.
Change-Id: I6cd9cc84898a858e73d13297c27eeeac28010d9e
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>