x86/fpu: Combine fpu_ctxt and xsave_area in arch_vcpu

x86/fpu: Combine fpu_ctxt and xsave_area in arch_vcpu

fpu_ctxt is either a pointer to the legacy x87/SSE save area (used by FXSAVE) or
a pointer aliased with xsave_area that points to its fpu_sse subfield. Such
subfield is at the base and is identical in size and layout to the legacy
buffer.

This patch merges the 2 pointers in the arch_vcpu into a single XSAVE area. In
the very rare case in which the host doesn't support XSAVE all we're doing is
wasting a tiny amount of memory and trading those for a lot more simplicity in
the code.

While at it, dedup the setup logic in vcpu_init_fpu() and integrate it
into xstate_alloc_save_area().

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>