Add support for KVM_GET_XSAVE2

[zulinx86]

Summary of the PR

Since Linux 5.17, the kvm_xsave struct has a flexiblre array member
(FAM) at the end, which can be retrieved using the KVM_GET_XSAVE2
ioctl [1]. What makes this FAM special is that the length is not stored
in itself, but has to be retrieved via
KVM_CHECK_CAPABILITY(KVM_CAP_XSAVE2) which returns the total size of
the kvm_xsave struct (e.g. the traditional 4096 byte region + extra
region with the size of the FAM). To support it in rust-vmm, Xsave has
been introduced as its FamStructWrapper.

The size required to hold the whole kvm_xsave structure can vary
depending on features that have been dynamically enabled by
arch_prctl() [2]. Any features must not be enabled after the size has
been confirmed; otherwise, KVM_GET_XSAVE2 writes beyond the allocated
area for Xsave, potentially causing undefined behavior. It is unable
to put KVM_CHECK_CAPABILITY call for the size check and
KVM_GET_XSAVE2 call together into get_xsave2(), because there is a
chance of race condition where another thread enables additional
features between them. That's why get_xsave2() is marked unsafe.

Although KVM_SET_XSAVE was extended and KVM_SET_XSAVE2 was not added
to support the kvm_xsave with FAM, set_xsave2() is also implemented
to enable users to pass Xsave to it for convenience. That is also
marked unsafe for the same reason.

In addition to that, after Linux 5.17, the existing set_xsave2() may copy data
beyond the traditional 4096 bytes if XSTATE features are dynamically enabled.
So it is also marked unsafe.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.