Extend max vcpu number for HVM guest

Extend max vcpu number for HVM guest

Reduce size of Xen-qemu shared ioreq structure to 32 bytes. This
has two advantages:
 1. We can support up to 128 VCPUs with a single shared page
 2. If/when we want to go beyond 128 VCPUs, a whole number of ioreq_t
    structures will pack into a single shared page, so a multi-page
    array will have no ioreq_t straddling a page boundary

Also, while modifying qemu, replace a 32-entry vcpu-indexed array
with a dynamically-allocated array.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>