improve x86's alloc_vcpu_guest_context()

improve x86's alloc_vcpu_guest_context()

This essentially reverts c/s 2037f2adb "x86: introduce
alloc_vcpu_guest_context()", including the newer arm bits, but achieves
the same end goal by using the newer vmalloc() infrastructure.

For both x86 and ARM, {alloc,free}_vcpu_guest_context() become arch-local
static inlines (which avoids a call into a separate translation),
and removes an x86 scalability limit when compiling with a large NR_CPUS.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>