KVM: x86: Use local MMIO fragment variable to clean up emulator_read_write()

KVM: x86: Use local MMIO fragment variable to clean up emulator_read_write()

Grab the MMIO fragment used by emulator_read_write() to initiate an exit
to userspace in a local variable to make the code easier to read.

No functional change intended.

Tested-by: Tom Lendacky <thomas.lendacky@gmail.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260225012049.920665-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>