libxc: rework BSP initialization

libxc: rework BSP initialization

Place the calls to xc_vcpu_setcontext and the allocation of the hypercall
buffer into the arch-specific vcpu hooks. This is needed in order to
introduce a new builder, so x86 HVM guests can initialize the BSP using
XEN_DOMCTL_sethvmcontext instead of XEN_DOMCTL_setvcpucontext.

This patch should not introduce any functional change.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>