util: mdev: Improve the error msg on non-existent mdev prior to VM start

util: mdev: Improve the error msg on non-existent mdev prior to VM start

What one currently gets is:
failed to read '/sys/bus/mdev/devices/<UUID>/mdev_type/device_api': No
such file or directory

This indicates that something is missing within the device's sysfs tree
which likely might be not be the case here because the device simply
doesn't exist yet. So, when creating our internal mdev obj, let's check
whether the device exists first prior to trying to verify the
user-provided model within domain XML.

Signed-off-by: Erik Skultety <eskultet@redhat.com>