qemu: Don't check for /dev/kvm presence

The file being present doesn't necessarily mean anything these
days, as it's created independently of whether the kvm module
has been loaded[1]; moreover, we're already gathering all the
information we need through QMP, so poking the filesystem at
all is entirely unnecessary.

[1] https://github.com/systemd/systemd/commit/d35d6249d5a7ed3228

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>