OvmfPkg/PlatformDebugLibIoPort: Reword QEMU to hypervisor

We are going to reuse PlatformDebugLibIoPort to use debug IO port from
hypervisors that aren't QEMU, so reword "QEMU" to "hypervisor" in the
descriptions.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200423095358.2518197-3-anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>