runqemu: fix undefined variable reference in check_arg_path()

runqemu: fix undefined variable reference in check_arg_path()

'arg' isn't defined, the right name there is 'p'.

This fixes a rather obscure error message when that code path
ends up being taken:

$ runqemu some/existing-file-name
runqemu - ERROR - name 'arg' is not defined
runqemu - ERROR - Try 'runqemu help' on how to use it

(From OE-Core rev: 3f11e4cbb36fc65ff92296065e5f0a508b210ac7)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>