qemu.bbclass: fix segfaults when running through pseudo

qemu user binaries sometimes segfault when running them through pseudo.
So, set PSEUDO_UNLOAD to 1 before running any qemu binary.

[YOCTO #3788]

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>