selftests: kvm: fix sync_regs_test with newer gccs

Commit 204c91ef ("KVM: selftests: do not blindly clobber registers in
 guest asm") was intended to make test more gcc-proof, however, the result
is exactly the opposite: on newer gccs (e.g. 8.2.1) the test breaks with

==== Test Assertion Failure ====
  x86_64/sync_regs_test.c:168: run->s.regs.regs.rbx == 0xBAD1DEA + 1
  pid=14170 tid=14170 - Invalid argument
     1	0x00000000004015b3: main at sync_regs_test.c:166 (discriminator 6)
     2	0x00007f413fb66412: ?? ??:0
     3	0x000000000040191d: _start at ??:?
  rbx sync regs value incorrect 0x1.

Apparently, compile is still free to play games with registers even
when they have variables attached.

Re-write guest code with 'asm volatile' by embedding ucall there and
making sure rbx is preserved.

Fixes: 204c91ef ("KVM: selftests: do not blindly clobber registers in guest asm")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>