KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

Implement a workaround for an SNP erratum where the CPU will incorrectly
signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
RMP entry of a VMCB, VMSA or AVIC backing page.

When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
backing   pages as "in-use" in the RMP after a successful VMRUN.  This
is done for _all_ VMs, not just SNP-Active VMs.

If the hypervisor accesses an in-use page through a writable
translation, the CPU will throw an RMP violation #PF. On early SNP
hardware, if an in-use page is 2mb aligned and software accesses any
part of the associated 2mb region with a hupage, the CPU will
incorrectly treat the entire 2mb region as in-use and signal a spurious
RMP violation #PF.

The recommended is to not use the hugepage for the VMCB, VMSA or
AVIC backing page. Add a generic allocator that will ensure that the
page returns is not hugepage (2mb or 1gb) and is safe to be used when
SEV-SNP is enabled.

Co-developed-by: Marc Orr <marcorr@google.com>
Signed-off-by: Marc Orr <marcorr@google.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

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