qemu: Fix mdev checking for VFIO support

qemu: Fix mdev checking for VFIO support

Commit a4a39d90 added a check that checks for VFIO support with mediated
devices. The problem is that the hostdev preparing functions behave like
a fallthrough if device of that specific type doesn't exist. However,
the check for VFIO support was independent of the existence of a mdev
device which caused the guest to fail to start with any device to be
directly assigned if VFIO was disabled/unavailable in the kernel.
The proposed change first ensures that it makes sense to check for VFIO
support in the first place, and only then performs the VFIO support check
itself.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1441291

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