qcow: fix signedness bugs

Some functions in qcow.c return u64, but are checked against < 0
because they want to check for the -1 error return value.
Do an explicit comparison against the casted -1 to express this
properly.
This was silently compiled out by gcc, but clang complained about it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>