x86emul: adjust handling of AVX2 gathers

x86emul: adjust handling of AVX2 gathers

HVM's MMIO cache only has a capacity of three entries. Once running out
of entries, hvmemul_linear_mmio_access() will return
X86EMUL_UNHANDLEABLE. Since gathers are an iterative process anyway,
simply commit the portion of work done in this and hypothetical similar
cases, exiting back to guest context for the insn to be retried.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>