KVM: x86: POPA emulation may not clear bits [63:32]

POPA should assign the values to the registers as usual registers are assigned.
In other words, 32-bits register assignments should clear bits [63:32] of the
register.

Split the code of register assignments that will be used by future changes as
well.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Message-Id: <1427719163-5429-3-git-send-email-namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>