gdb: snapshot restore breaks breakpoints #1972

• Hardware breakpoints are lost on MSHV and WHP. Those backends store gdb hardware breakpoints in the guest's DR0-DR7 (mshv/x86_64.rs#L853, whp/x86_64.rs#L1181). reset_vcpu zeroes those registers (x86_64.rs#L327). gdb still lists the breakpoints. KVM keeps them in kvm_guest_debug, so they survive there.
• Software breakpoints are written into snapshot memory. gdb builds make snapshot memory writable for this (build.rs#L162).
  • snapshot() captures the 0xCC bytes. After restore nothing tracks them.
  • restore() can remove a breakpoint that sw_breakpoints still lists. Removing it later writes the saved byte over whatever the restored snapshot holds at that address.

Possible fix: keep breakpoint state on the host and apply it again after restore(). Take software breakpoints out before snapshot().