qemu_find_file: check name as a straight path even if it has no '/'

Make qemu_find_file() check for the passed in name as a straight
pathname even if it doesn't have any path separator character in it.
This means that "-bios foo", "-dtb foo" etc will find a file 'foo'
in the current directory.
This removes an inconsistency with -kernel and -initrd, which both
accept plain filenames as meaning files in the current directory.
It's also less confusing for the user than an undocumented restriction
that "this option accepts a filename, except for the special case
where the filename you pass happens not to have a '/' in it, in
which case we'll ignore it."

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
