runqemu: Let qemuparams override default settings

runqemu: Let qemuparams override default settings

Fixed:
In meta/conf/machine/include/qemuboot-x86.inc:
QB_CPU_x86-64 = "-cpu core2duo"

$ runqemu qemux86-64 qemuparams="-cpu coreduo"
Check /proc/cpuinfo, it should use coreduo rather than core2duo since user
specifies it, but it doesn't, append qemuparams to the last can fix the
problem.

[YOCTO #11773]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>