Support QEMU firmware descriptor file with memory device

Recent QEMU package from distros contain new format of firmware descriptor for stateless firmware.

```
$ cat /usr/share/qemu/firmware/60-edk2-ovmf-x64-amdsev.json
{
    "description": "OVMF with SEV-ES support",
    "interface-types": [
        "uefi"
    ],
    "mapping": {
        "device": "memory",
        "filename": "/usr/share/edk2/ovmf/OVMF.amdsev.fd"
    },
    "targets": [
        {
            "architecture": "x86_64",
            "machines": [
                "pc-q35-*"
            ]
        }
    ],
    "features": [
        "amd-sev",
        "amd-sev-es",
        "amd-sev-snp",
        "verbose-dynamic"
    ],
    "tags": [

    ]
}
```

However current logic within nova to look up firmware files for UEFI boot instances do not support this format and always assumes flash device.
This bp aims to make sure nova can handle this format correctly so that we can support features which requires the memory device firmware (AMD SEV-SNP and Intel TDX)