Probe host capabilities

[jpbrucker]

@anta5010 suggests adding a probing function to the tool, that finds out VM capabilities (PMU, BPs/WPs, SVE, but also default CPU number, GIC versions etc needed for FW table generation). It can be useful for at least two reasons:

• we wouldn't have to pass fixed config files when in "generate DTB" mode, before launching the VM
• we could also generate a config file for offline measurements.

KVM doesn't yet export the RMM features register to userspace, but it does provide default values of ID registers with GET_ONE_REG. Getting some features from host platform registers is possible, but KVM and RMM can decide for whatever reason to use different values for guests, so probing by creating a scratch VM is probably the most accurate method at the moment.

[anta5010]

Please notice that the realm params have been removed from RIM calculations since rmm1.1alp16: https://jira.arm.com/browse/FENIMORE-905

I'm yet to find out when the changes will be implemented in rmm code. If it happens soon then this issue will be obsolete and we will need to update realm-measurement with the new functionality.

[jpbrucker]

Right. I suspect detecting the host features will still be useful as RMM gets more complex and VMMs do a lot of auto-detection of features (For example #2 gic-version=host in QEMU in theory would require knowing host capabilities although in practice it's only GICv3 at the moment. Or for example default vCPU count in kvmtool, which we need to know for the DT, is the host's CPU count).

Even knowing what RMM version we're on would be useful to know what gets measured, but KVM doesn't yet provide that information