mshv: Add SVM_EXIT_CPUID, SVM_EXIT_MSR and GHCB_MSR_TERM_REQ handlers for SEV-SNP - #7934

When booting an SEV-SNP guest VM using IGVM with -pvalidate_opt 1 (lazy
page acceptance), the guest kernel's #VC exception handler may issue
VMGEXIT with SVM_EXIT_CPUID (0x72) or SVM_EXIT_MSR (0x7c) exit codes
via the GHCB page protocol. The hypervisor had no handlers for these
exit codes, causing the guest's #VC handler to fail and trigger
sev_es_terminate(), which sends GHCB_MSR_TERM_REQ (0x100). The
hypervisor then panicked on the unhandled 0x100 operation.

Add the following handlers to the GHCB VMGEXIT processing:

- SVM_EXIT_CPUID (0x72): Read function/index/xcr0/xss from the GHCB
  page and return CPUID results via get_cpuid_values().
- SVM_EXIT_MSR (0x7c): Handle MSR read (RDMSR) and write (WRMSR)
  requests from the guest via the GHCB page protocol.
- GHCB_MSR_TERM_REQ (0x100): Decode reason_set and reason_val from the
  GHCB MSR and return an error instead of panicking, allowing graceful
  error propagation.

Testing:

  Reproducer (on Azure DC16as_cc_v5, /dev/mshv):

  cloud-hypervisor --cpus boot=1,nested=off --memory size=512M \
    --disk path=osdisk.img path=cloudinit \
    --net "tap=,mac=12:34:56:78:90:06,ip=192.168.6.1,mask=255.255.255.128" \
    --serial null --console pty \
    --api-socket /tmp/ch.sock \
    --igvm /igvm_files/linux-ttyS0.bin \
    --host-data <hash> --platform sev_snp=on -v

  Before fix:

    thread 'vcpu0' panicked at hypervisor/src/mshv/mod.rs:1207:30:
    Unsupported VMGEXIT operation: 100

  After fix: VM boots successfully to login prompt with no panics.
  All virtio devices (console, rng, disks) activate normally.

No regression risk for non-SEV-SNP: all new code is within the
HVMSG_X64_SEV_VMGEXIT_INTERCEPT handler which is only reached for
SEV-SNP guests.

Signed-off-by: Souradeep Chakrabarti <[email protected]>