pc-bios/descriptors: fix paths in json files

pc-bios/descriptors: fix paths in json files

Before the change /usr/share/qemu/firmware/50-edk2-x86_64-secure.json
contained the relative path:
            "filename": "share/qemu/edk2-x86_64-secure-code.fd",
            "filename": "share/qemu/edk2-i386-vars.fd",

After then change the paths are absolute:
            "filename": "/usr/share/qemu/edk2-x86_64-secure-code.fd",
            "filename": "/usr/share/qemu/edk2-i386-vars.fd",

The regression appeared in qemu-5.2.0 (seems to be related
to meson port).

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: "Marc-André Lureau" <marcandre.lureau@redhat.com>
CC: "Philippe Mathieu-Daudé" <philmd@redhat.com>
Bug: https://bugs.gentoo.org/766743
Bug: https://bugs.launchpad.net/qemu/+bug/1913012
Signed-off-by: Jannik Glückert <jannik.glueckert@gmail.com>
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Message-Id: <20210131143434.2513363-1-slyfox@gentoo.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>