xen: arm: improve VMID allocation.

xen: arm: improve VMID allocation.

The VMID field is 8 bits. Rather than allowing only up to 256 VMs per host
reboot before things start "acting strange" instead maintain a simple bitmap
of used VMIDs and allocate them statically to guests upon creation.

This limits us to 256 concurrent VMs which is a reasonable improvement.
Eventually we will want a proper scheme to allocate VMIDs on context switch.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>