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

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

Initializing an unsigned as -1, or using temporary
sval for conversion is awkward.  Since we don't allow
other "negative" values anyway, use signed value and
pass it to bdrv_img_create() (where it is properly
converted to unsigned), simplifying code.

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: <20250531171609.197078-3-mjt@tls.msk.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>