This patch lets GCC to determine which registers to save when we
switch to/from a VCPU in the case of intel x86_64.

* Original code saves following registers:

    rax, rbx, rcx, rdx, rsi, rdi, rbp,
    r8, r9, r10, r11, r12, r13, r14, r15

* Patched code:

  - informs GCC that we modify following registers
    using the clobber description:

    rbx, rdi, rsi,
    r8, r9, r10, r11, r12, r13, r14, r15

  - doesn't save rax because it is an output operand (vmx->fail)

  - cannot put rcx in clobber description because it is an input operand,
    but as we modify it and we want to keep its value (vcpu), we must
    save it (pop/push)

  - rbp is saved (pop/push) because GCC seems to ignore its use in the clobber
    description.

  - rdx is saved (pop/push) because it is reserved by GCC (REGPARM) and
    cannot be put in the clobber description.

  - line "mov (%%rsp), %3 \n\t" has been removed because %3
    is rcx and rcx is restored just after.

  - line ASM_VMX_VMWRITE_RSP_RDX() is moved out of the ifdef/else/endif
Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>