Improve error reporting when -drive format= is incorrect

[Cole Robinson]

With qemu.git as of e376a788ae130454ad5e797f60cb70d0308babb6

$ qemu-img create -f raw test.raw 1M
Formatting 'test.raw', fmt=raw size=1048576
$ ./x86_64-softmmu/qemu-system-x86_64 -drive file=./test.raw,format=qcow2
qemu-system-x86_64: -drive file=./test.raw,format=qcow2: could not open disk image ./test.raw: Invalid argument
$ ./x86_64-softmmu/qemu-system-x86_64 -drive file=./test.raw,format=vdi
qemu-system-x86_64: -drive file=./test.raw,format=vdi: could not open disk image ./test.raw: Operation not permitted

Ideally we'd get something like
could not open disk image ./test.raw: not a qcow2 format file

(this was kicking around in Fedora bugzilla for a long time at https://bugzilla.redhat.com/show_bug.cgi?id=556482 )

[Stefan Weil (ubuntu-weilnetz)]

Fixed in QEMU 1.4