OvmfPkg/LinuxInitrdDynamicShellCommand: bail if initrd already exists

OvmfPkg/LinuxInitrdDynamicShellCommand: bail if initrd already exists

Before taking any actions, check if an instance of the LoadFile2 exists
already on the Linux initrd media GUID device path, and whether it was
provided by this command. If so, abort, since no duplicate instances of
the device path should exist.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2564
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>