exit if -drive specified is invalid instead of ignoring the "wrong" -drive

This fixes the problem when qemu continues even if -drive specification
is somehow invalid, resulting in a mess.  Applicable for both current
master and for stable-0.14 (and the same issue exist 0.13 and 0.12 too).

The prob can actually be seriuos: when you start guest with two drives
and make an error in the specification of one of them, and the guest
has something like a raid array on the two drives, guest may start failing
that array or kick "missing" drives which may result in a mess - this is
what actually happened to me, I did't want a resync at all, and a resync
resulted in re-writing (and allocating) a 4TB virtual drive I used for
testing, which in turn resulted in my filesystem filling up and whole
thing failing badly.  Yes it was just testing VM, I experimented with
larger raid arrays, but the end result was quite, well, unexpected.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
