qemuboot.bbclass: make sure runqemu boots bundled initramfs kernel image

The QB_DEFAULT_KERNEL is set to pick bundled initramfs kernel image
if the Linux kernel image is generated with INITRAMFS_IMAGE_BUNDLE="1".

This makes runqemu to automatically pick bundled initramfs kernel image
instead of explicitly mentioning bundled initramfs kernel image in
runqemu.

[YOCTO #14748]

Signed-off-by: Jagadeesh Krishnanjanappa <workjagadeesh@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>