qemu.bbclass: add qemu_wrapper_cmdline()

qemu.bbclass: add qemu_wrapper_cmdline()

The class had qemu_run_binary() which was not suitable for gobject-introspection,
as it required the name of the binary to run.

qemu_wrapper_cmdline() returns just the command line string needed to run
binaries, and does not require the binary name.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>