tools/pvh: set coherent MTRR state for all vCPUs

tools/pvh: set coherent MTRR state for all vCPUs

Instead of just doing it for the BSP. This requires storing the
maximum number of possible vCPUs in xc_dom_image.

This has been a latent bug so far because PVH doesn't yet support
pci-passthrough, so the effective memory cache attribute is forced to
WB by the hypervisor. Note also that even without this in place vCPU#0
is preferred in certain scenarios in order to calculate the memory
cache attributes.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>