boards: xenvm: return separate defconfig for xenvm with GICv3

During migration to HWMv2 separate specific defconfig file was removed
for GICv3 version of virtual xenvm boards. It worked fine before
commit 0be0d217 ("cmake: modules: extensions: Revert using common
board files") significantly changed build behavior, but did not return
previously removed file. This led to build/runtime issues, when some of
the Kconfig options were not selected.

Return GICv3 specific defconfig to board directory to fix configuration
problems.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>