qemu-img: measure: convert img_size to signed, simplify handling

qemu-img: measure: convert img_size to signed, simplify handling

qemu_opt_set_number() expects signed int64_t.

Use int64_t instead of uint64_t for img_size, use -1 as "unset"
value instead of UINT64_MAX, and do not require temporary sval
for conversion from string.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250531171609.197078-2-mjt@tls.msk.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>