target/i386: disable/enable breakpoints on vmentry/vmexit

If the required DR7 (either from the VMCB or from the host save
area) disables a breakpoint that was enabled prior to vmentry
or vmexit, it is left enabled and will trigger EXCP_DEBUG.
This causes a spurious #DB on the next crossing of the breakpoint.

To disable it, vmentry/vmexit must use cpu_x86_update_dr7
to load DR7.

Because cpu_x86_update_dr7 takes a 32-bit argument, check
reserved bits prior to calling cpu_x86_update_dr7, and do the
same for DR6 as well for consistency.

This scenario is tested by the "host_rflags" test in kvm-unit-tests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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