xvisor: fix build on AArch64

xvisor: fix build on AArch64

Xvisor was failing to build on AArch64 with:

package/xvisor/xvisor.mk:60: *** No Xvisor defconfig name specified, check your BR2_PACKAGE_XVISOR_DEFCONFIG setting.  Stop.

The first problem is that the Config.in file had a typo: it was using
BR2_AARCH64 instead of BR2_aarch64, and therefore the
BR2_PACKAGE_XVISOR_DEFCONFIG variable had no value.

Once this is fixed, another problem occurs: the ARCH variable needs to
be specified as "arm" for XVisor, for both ARM and AArch64. Therefore,
a XVISOR_ARCH variable is introduced, which is calculated according to
the Buildroot configuration options. Only x86-64, arm and aarch64 are
supported by Xvisor currently, so it remains simple.

Fixes:

  http://autobuild.buildroot.net/results/1719a63ff257f13634a06a14327abfb327984101/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>