tools/libxl: search PATH for QEMU if `QEMU_XEN_PATH` is not absolute

tools/libxl: search PATH for QEMU if `QEMU_XEN_PATH` is not absolute

`QEMU_XEN_PATH` will be configured as `qemu-system-i386` with no clue where, if
`--with-system-qemu` is set without giving a path (as matched in the case `yes`
but not `*`). However, the existence of the executable is checked by `access()`,
that will not look for anywhere in $PATH but the current directory. And since it
is possible for `qemu-system-i386` (or any other configured values) to be
executed from PATH later, we'd better find that in PATH and return the full path
for the caller to check against.

Signed-off-by: Hongbo <hehongbo@mail.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>