xen: move the xenvm machine to mach-virt

xenvm is based on mach-vexpress, move it to mach-virt.

Changes in v4:
- update the dts Makefile too.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Marc Zyngier <marc.zyngier@arm.com>
CC: will.deacon@arm.com
CC: arnd@arndb.de
CC: rob.herring@calxeda.com