qemuboot-x86: move QB_SYSTEM_NAME to corresponding conf

qemuboot-x86: move QB_SYSTEM_NAME to corresponding conf

Configrations:
MACHINE: qemux86-64
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

Reproduce steps:
bitbake lib32-core-image-minimal
runqemu qemux86-64 nographic lib32-core-image-minimal

Errors:
qemu cannot bootup since:
Booting from ROM...
This kernel requires an x86-64 CPU, but only detected an i686 CPU.
Unable to boot - please use a kernel appropriate for your CPU.
QEMU: Terminated

For lib32 image, override has x86, so the qemubin set to qemu-system-i386,
fix by move QB_SYSTEM_NAME to corresponding conf, don't use the override

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>