libxl: move check for existence of qemuu device model

libxl: move check for existence of qemuu device model

The stat in libxl__domain_build_info_setdefault's default device model
logic works to fall back to qemu-xen-traditional whenever the
executable for qemu-xen is not found.

We are going to use qemu-xen-traditional in more cases, so break this
check out into its own if statement.

Also add a pair of braces to make the if() statement symmetrical.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>