vm: fix deadlock in UnusedTCPPort #14979

Job	Run time
build	4m 26s
arch (presubmit_arch_linux)	10m 48s
aux	4m 51s
dashboard	5m 33s
arch (presubmit_arch_freebsd)	6m 55s
arch (presubmit_arch_netbsd)	4m 26s
gvisor	4m 21s
old	6m 49s
arch (presubmit_arch_openbsd)	4m 20s
race	9m 34s
arch (presubmit_arch_darwin)	4m 21s
race_dashboard	6m 46s
arch (presubmit_arch_windows)	3m 23s
arch (presubmit_arch_executor)	3m 14s
	1h 19m 47s

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