hw/virtio/virtio-pci: Avoid compiler warning with -Wshadow

hw/virtio/virtio-pci: Avoid compiler warning with -Wshadow

"len" is used as parameter of the functions virtio_write_config()
and virtio_read_config(), and additionally as a local variable,
so this causes a compiler warning when compiling with "-Wshadow"
and can be confusing for the reader. Rename the local variables
to "caplen" to avoid this problem.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231004095302.99037-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>