x86/pv: Fix the handing of writes to %dr7

x86/pv: Fix the handing of writes to %dr7

c/s 65e35549 "x86/PV: support data breakpoint extension registers"
accidentally broke the handing of writes.  The call to activate_debugregs()
doesn't write %dr7 as v->arch.debugreg[7] hasn't been updated yet, and the
break skips the intended write to %dr7.

Remove the break, causing execution to hit the write_debugreg(7, value); in
context at the bottom of the hunk, which in turn causes hardware to be updated
appropriately.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>