xen/virtual-region: Rename the start/end fields

xen/virtual-region: Rename the start/end fields

... to text_{start,end}.  We're about to introduce another start/end pair.

Despite it's name, struct virtual_region has always been a module-ish
description.  Call this out specifically.

As minor cleanup, replace ROUNDUP(x, PAGE_SIZE) with the more concise
PAGE_ALIGN() ahead of duplicating the example.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
master commit: 989556c6f8ca080f5f202417af97d1188b9ba52a
master date: 2024-03-07 14:24:42 +0000