x86/io: rename misleading dpci_ prefixed functions to g2m_

x86/io: rename misleading dpci_ prefixed functions to g2m_

The dpci_ prefix used on those IO handlers is misleading, there's nothing PCI
specific in them, they simply map a guest IO port into a machine (physical) IO
port. They don't specifically trap the PCI IO port range in any way
(0xcf8/0xcfc).

Rename them to use the g2m_ prefix in order to avoid this confusion.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>