Replce the literal magic number 48 with length calculation (32 bytes at
the end of the firmware after the table footer + 16 bytes of the OVMF
table footer GUID).

No functional change intended.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220222071906.2632426-3-dovmurik@linux.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>