virtual_media: use configured manager URI name

The VirtualMedia handlers compared the manager path segment against
a hardcoded "bmc" instead of BMCWEB_REDFISH_MANAGER_URI_NAME. That
constant comes from the redfish-manager-uri-name meson option,
which defaults to "bmc" but may be set to any value.

When the option is set to another value, Manager still advertises
VirtualMedia at /redfish/v1/Managers/<name>/VirtualMedia, but the
VirtualMedia handlers rejected that URI with resourceNotFound.
The collection GET, the resource GET, and the InsertMedia and
EjectMedia actions were all unreachable, so Manager published a
dangling link.

Compare against BMCWEB_REDFISH_MANAGER_URI_NAME instead, matching
how managers.hpp, network_protocol.hpp, and update_service.hpp
already validate the manager segment. Behavior is unchanged for
the default option value.

Tested:

Built bmcweb with -Dredfish-manager-uri-name=manager.
Verified GET /redfish/v1/Managers/manager/VirtualMedia works.
Verified the default manager URI behavior remains unchanged.

Change-Id: I2375398d6a2780ef403870061d6583b3ae699551
Signed-off-by: Rython Cai <rython_cai@jabil.com>