qdev: fix OVERFLOW_BEFORE_WIDEN

Potentially overflowing expression "1 << prop->bitnr" with
type "int" (32 bits, signed) is evaluated using 32-bit arithmetic,
and then used in a context that expects an expression of type
"uint64_t" (64 bits, unsigned).

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>